The simplest puzzle I know of... two identical pieces forming a hollow 30mm cube, yet surprisingly difficult to take apart when you don't know how it fits together. I've supplied the puzzle in three different wall thicknesses, each one is interesting. The puzzle can be scaled up.
Print at 0.1 mm or 0.2 mm, either one will work. No support, 15% in-fill (for the thicker walled puzzle).