When I have an idea for something neat I would like the players to do, I sometimes tie it in as a subplot of my current campaign. A few years ago I had an idea for playing some sessions as a mystery. They had to prove the innocence of a man that could not be divined. It was impossible for magic to determine his guilt or innocence as no divination spell would work on him. They players were forced to do some good old detective work to figure it out. I tied him into their current campaign by making him a must have traveling companion. His non divination made him a living walking amulet of non-detection, something they needed to avoid the bad guys chasing them. So the players ended up over a series of sessions proving this guys innoncence and getting him to travel with them for awhile. The fact that they invested so much time in him tied him to the group pretty well that they made sure he landed on his feet when they got done with him.