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?
After the PCs defeat (or force the rapid retreat of) a villanous necromancer/demon summoner, they discover a book. This book outlines how to summon a demon whose power increases according to the size of the summoning circle used to summon it. And after the PCs examine a map of the country, they find that the layout of the cities and roads match up with the required summoning circle. In fact, the final road that would complete the circle is currently in construction.
With anyone who can find out about the circle and the ritual to summon the demon able to somehow make the final road/line, and destruction of the cities and roads currently in existence out of the question (unless this is an evil campaign- that removes the moral quandary), how are the PCs going to solve this problem?