Someone always needs a favor. If you’re ever looking for your next adventure, there’s an NPC somewhere—that the players have already talked to—that likely needs something. It could be a simple fetch quest that they can’t do. Maybe the location is really far away, hard to traverse, or dangerous for them. Could be that they are just too busy. Perhaps what they need isn’t theirs for the taking and you’ll need to liberate it from someone.
Another possibility is that they need you to take them somewhere. They need to get to the town of Brook’s Shoal by the end of the week and they’re carrying something that someone wants enough to kill them. Well, now you have to go fast, but you’re being harried and you don’t know the terrain, sounds like a surefire way to get in some fun trouble.
One more: your mentor’s dying wish as they died in front of your eyes was for you to reclaim their ancestral armor that was stolen many years ago by a lowly sergeant in the same army. Well, turns out that sergeant kept busy and is now the a general of an opposing army and she always wears that armor into battle. Good luck getting that back.