A Magical Puzzle Key for opening a door. Innocuous, somewhat simple, but fun to solve, if you like puzzles.
A magical barrier of acid blocks your path. Solve the puzzle to unlock the door.
A combination lock, possibly to open a chest or door or other entry point. A magical puzzle, for those interested.
Checkerboards on floors of dungeons… is there any greater bane to adventurers?
Believable magic, like ordinary physicis, operates according to some invariable laws that always result in some kind of cost or "bounce back". The grater the magic, the more it should cost the character physically, emotionally, and spiritually.