I make high quality Lego Puzzle Boxes and instructions.
My three rules for building puzzle boxes are:
1. All parts needed to solve a puzzle box have to be contained inside the box.
2. No pieces can be directly taken apart. They may be indirectly pulled apart though so you could have an axle sticking out of a hole and when someone pulls on the axle a bush falls off the end that is hidden.
3. You can push, pull, slide or turn pieces to try to solve the puzzle box.
Check out SoulPancake's video on a puzzle box if you want to know what a puzzle box is.