Traps and puzzles

Not for everyone

Before we delve into the more nitty gritty stuff it's important to consider the group you're playing with. Not everybody enjoy traps, let alone puzzles, but much of this does depend on the types of traps and puzzles you throw at them. But both traps and puzzles can be a time sink and a distraction from the overall story. Figuring out the solution to a puzzle can take far longer than you expected, a single trap can turn into a frantic search for more traps around every corner, and some people might simply not be the greatest puzzlers. No matter the reason it's important to take into account what your players want, but that should go without saying.

The biggest issue with traps and puzzles is that they can often seem unfair, especially traps. It can be a careful balance to give the players enough hints to figure things out, without holding their hands to lead them through it every step of the way. But when some information is omitted, it can come across as cheating, while giving all the information right away defeats the whole point of putting traps and puzzles in your campaign in the first place. So some game masters (GMs) prefer to leave out traps and puzzles entirely, but that's a measure you don't necessarily have to take.

They're encounters

The first thing to realize when trying to make traps and puzzles more interesting is that you need to consider them to be encounters. A simple pressure plate activating a crossbow somewhere is pretty simple and straight forward, but, besides the damage it may or may not have done, it doesn't add much to the campaign.
Like every other type of encounter they work way better if they add more than just a simple obstacle. Encounters can be story driven, nerve wrecking, offer role play opportunities, raise the stakes, and so much more.

When you add a trap or puzzle you need to consider who put it there, how it's created, how it's concealed, skill level of the trap builder, what's the intention of the trap (hunt, kill, maim, alarm, deter, etc.), how old and reliable is it, and so on, and so on.
A hunter might put a bear trap down to hunt for a bear, a kobold might put traps down to deter other beings from entering its lair, a mighty wizard might put a magical puzzle in place to test the skills of those who dare seek their secrets, and a malevolent deity may give the group a puzzle riddled with traps to try and rig the results.

There are so many story driven elements traps and puzzles can add, it'd be a shame not to make use of such opportunities. But it also helps with making traps seem more fair, and with puzzles more solvable.
Traps are usually put down by some being, a being which will need to know where their own traps are in order to avoid triggering them themselves in the future, so there may be little clues the party could discover to avoid future traps. Traps also cost resources, so a wide variety of traps may not always be an option, especially if the location doesn't allow for this. Good luck digging a pitfall trap in solid rock for example.
Puzzles may have been attempted by other beings in the past as well, beings who've left traces and clues for a struggling party to discover, or beings who've tried to break the puzzle, and perhaps almost succeeded.

Turn traps into puzzles

A very effective way of dealing with traps is to turn them into puzzles. The individual trap doesn't have to be one, but the traps overall definitely could be, or at least the process of figuring them out could be. The locations of traps, the way they're constructed, the way they're overcome, and so on, these can all be part of a larger puzzle.
For example, let's say a party of adventurers is making their way through a forest, but suddenly one of the party members triggers a trap, which sends a log on ropes swinging their way. The trap was triggered by stepping on a pile of suspicious looking leaves, so similar piles can be an obvious clue, but they could also pose a more subtle clue of having to look for other suspicious looking natural elements. Bushes, twigs, vines, anything could be a trigger. The same can be done for all locations, and in different ways too.

You can extend this to far larger reaching puzzles, but you can intertwine more story driven element, or more universe building elements too. That tree on ropes could be a fallen tree tied to vines so no natural elements are wasted or overly disturbed, hinting that this trap was made by beings who respect nature. The poison used in darts could provide a clue as to who put the trap there, a broken trap in a temple could indicate the age of the temple, and traps which don't match the surrounding area indicate they were placed by outsiders.

Individual traps can also be turned into puzzles however. The classic treasure on a stone trap is an example of that. Lift the treasure and the stone beneath it will rise, triggering a different part of the trap. Replace the treasure with something of equal weight or heavier, and you prevent the trap from triggering.
By creating or adapting traps this way the players will likely still have to make a roll to see if they succeed, but that's exactly the element which'll make the trap seem more fair. Being able to act on a trap is usually more fair than simply triggering it because you either missed a clue in the GM's description, or the GM simply omitted it to make sure you fell for the trap.

Reflexes

If a trap cannot be turned into a puzzle you could still allow the players to react before they deal with the consequences. Not seeing a pressure plate doesn't mean you wouldn't be able to jump out of the way of the swinging blades released by it for example, so if a character does trigger a trap you could allow the player to decide how their character responds. They'd have minimal information of course, sometimes only a mere "You triggered something beneath your feet, what do you do?" will be all they get, but it both takes away from the unfairness, and adds to the realism of the encounter.
You could even make the player roll a dice, and give them information based on their roll. A good roll might let them know they triggered something, as well as a sound coming from a direction for example, while a bad roll might merely let them know they felt something shift beneath their feet.

More than just damage

The consequences of failing a trap or puzzle can be far more than just damage. They could set of an alarm, block a path, cast a curse, mark a thief, and so on. These elements are usually also more fun to deal with for players, as they require you to think differently and find solutions to these problems, rather than simply having to deal with less health.
In a campaign a friend of mine and I wrote together, we had a magical library in which anybody who spoke too loudly would be shocked automatically. It was definitely part of that location and universe, and it made the players change the way they went through that library, as well as how they dealt with anybody they came across in that library. But we (and our players) also thought it was just a funny element to add, so it was a win-win all around.

Multiple solutions

Whenever you add a puzzle you'll want to make sure it can be solved in different ways, or make sure solving the puzzle isn't crucial to progress the campaign. If the puzzle can only be solved one way you risk the party not being able to figure it out, which would jeopardize the entire campaign. You could give clue after clue after clue, but sometimes people might over think things, or confuse each other with different theories. Plus explaining a puzzle is the last thing you want to do in a campaign, it ruins the entire flow, and it kind of defeats the point of planning a puzzle.

Not finding a solution is not a big deal if the puzzle doesn't prevent access to the crucial elements though. The puzzles could simply hide bonus loot, an alternative path, or something else which might help the party slightly. Still, it's usually far more satisfying for the group to overcome a puzzle, they are supposed to be heroic in some form after all.

The search issue

An issue you might face with traps is that the party ends up searching for traps in every location. This slows the game down immensely, and is just a boring way to play the game. You can spice it up a little with in depth descriptions of the searching party, but in the end nothing happens when there are no traps to be found.
To solve this you'll want to dissuade the party from searching around every corner, which is where the hints I used in the forest example earlier come in handy. If the players will only search for traps whenever you briefly mention specific elements you prevent the game from slowing down too much, and it makes sure the players pay attention to what you say.

Another way of doing it is by making each search boring, perhaps by dragging out the descriptions, or by making things repetitive, but you generally don't really want to add boring elements to a campaign.
Instead you could make the time they spend on searching for traps a detriment to their safety. Chasing monsters, a looming deadline, or perhaps finding and/or disarming the traps costs them resources other than just time (spells, etc.). There's plenty of ways to raise the stakes.

Rewards & xp

Of course there's also the issue of rewards. Not everything needs to offer rewards of course, but puzzles and traps do offer some methods of rewarding the characters and players. The obvious method is simply the treasures hidden behind the puzzles and traps, the great artifact at the end of the tomb, but the traps and puzzles could be rewards themselves.
The poisoned darts in an arrow trap may be salvaged, valuable components could be sold, beings caught and killed by traps might have possessions on them, the knowledge of how to construct a trap, or the knowledge of what traps a specific culture uses might make for knowledge a scholar is willing to pay for, and figuring out how a magical trap works could grant a sorcerer knew knowledge of spells they've never used.

Creating them

The biggest challenge might well be creating the actual traps and puzzles, but it doesn't have to be all that difficult. There are hundreds of puzzles you can adapt from real life, some of which I've already adapted, and can be found here.
Traps are usually far easier as they don't always require full details in terms of how they work, but even when they do it's usually just a case of figuring out the location first, and then using what knowledge the beings putting down the traps have to figure out what traps could be there.
For example, simple kobolds would likely use simple pitfalls, tripwires, and similar methods as traps. Whereas a clever wizard might use spells to animate inanimate objects once something is triggered, use runes instead of pressure plates, create illusionary walls, and so on.

A simple way of adding traps is to simply ask yourself: How would I rig this area with traps? Then alter what you would do to fit the intelligence of the beings putting down the traps, their available resources, the time they have to put down traps, and so on. But what if the being putting down the traps is supposed to be way more intelligent than you are? In that case they're likely also more intelligent than your group, which means players might not be able to figure out how to avoid the traps, which brings is back to the problems of unfairness from earlier. So keep things simple, that way the game doesn't stagnate. Besides, a truly intelligent being probably wouldn't bother with traps which can be avoided, disarmed, or at the very least not triggered on command.