Moon or Sun

Moon or Sun is a Nikoli puzzle in which the aim of the game is to draw a single, continuous loop on a grid divided by rooms. The grid can be any size, but the bigger the grid the bigger the challenge becomes. A puzzle is never all that difficult however.

The rules are as follows:
- Draw a single, continuous loop that goes through each room on the grid once. A room is marked by thicker lines.
- The loop can only go vertically and horizontally, it can never cross itself or branch off of itself.
- The loop has to go through either all moons or all the suns in a room, but cannot pass through both of them.
- Once the loop goes through one of the symbols in one room it has to go through the other symbol in the next.

Standard version

Moon or Sun is a very easy puzzle to solve. All you have to do is start in any given (small) room, start the loop and see if you ever end up at an impossible passage. If you do you know you have to start with the other symbol in the room you started.
This does become trickier when the grid is larger and there are more rooms connected to each other room, as this increases the amount of possible routes. In this case it's usually best to start at a cluster of symbols, as these often prevent certain moves from being made within those rooms.

Converting it to an RPG setting.

Moon or Sun can be a fun puzzle to adapt to an RPG setting. You could turn it into making sure each room has the right kind of power source, connect a long loop of magical energies, divide houses or fields of land to 2 different factions, figure out a safe route in a trap riddled building, or the path a security guard takes every hour.
Even just playing around with which two symbols you use and the look of the different rooms can be enough to adapt this puzzle to your own story universe.

In my example I went for an electricity themed puzzle. My character finds herself trapped in a room, there are giant minus and plus symbols on the floors and stepping on them causes her body to tingle with electric energy.
She isn't told anything else, so she'll have to explore the rooms to figure out how to solve this puzzle.

She'll quickly figure out that stepping on one symbol and then on the same one in the next room causes her to be shocked, which deals damage. Stepping on one symbol and then the other in the next room doesn't. So as long as she continues this pattern she'll eventually walk a full loop, which in turn solves the puzzle. A full loop will mean a fully connected power line, which will open up a hidden door leading to whatever comes next.

My example puzzle is a very simple one. Each room only leads to one possible other room. If you want to increase the time it takes to solve this puzzle try and make sure to have some rooms lead to 2 or more possible rooms. It usually still won't take too long to solve it, but to some every minute helps.
Alternatively, only reveal each room as the characters enter them. You could even add monsters, traps or other elements the players have to overcome if you really want to make this puzzle last. Perhaps the different symbols give different power ups.