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?
The player characters, experienced and somewhat well known, hear rumors and travelers' tales about a distant area being overrun by dragons (or other terrifying monstrosities). The locals have sent them a message, begging for heroic aid.
When they investigate, they discover that nothing of the sort is going on. It turns out that a group of thieves wanted them out of the way so that they could rob them (or someone who would normally receive their protection).