I used to set up geocaches and Easter egg hunts that involved puzzles. Generally, I start at the end point and work my way to the starting point . Depending upon how long you want it to last you should have between 5–10 problems/puzzles to solve. If there are too many, they must be simple and that gets boring and predicable. Too few and they must be complicated and that gets frustrating. Use light, darkness, UV, sound, to help find the clues. Make one puzzle tedious; one that requires special equipment they have already found (e.g. UV flashlight), one that takes special knowledge, and a few searching clues. Then throw in a few easy ones for kids. That is a nice mix. It sounds like fun!