I like to keep a couple of small side "adventures" handy to give players something to do that doesn't owe anything to the main story arc of the campaign. These are even smaller in scope than the "side trek" concept, but seem to add a lot to the campaign and the idea of the campaign setting as a living, breathing place. Here's a couple of examples:
Word gets around town about a magic item one of the PCs has recovered from the dungeon, and it is recognized as a family heirloom by a wealthy, influential member of the local citizenry. The local files a complaint in the courts, attempting to have the PCs arrested and the artifact confiscated. If the PCs handle this situation reasonably, the citizen may offer a replacement item, or better yet, provide them with a treasure map. Handled badly, outlawry or imprisonment could result.
The Dragon of the Deeps
A giant sturgeon (the average PC level +4HD) has been terrorizing fishermen in a local river or lake. The fishermen offer a reward for the PCs to hunt down and slay the beast.
A fighter of a level equal to the highest level PC+1 has heard of the PCs exploits and is jealous. He has traveled from far away to challenge the PC and assert his bragging rights. He will challenge the PC to a duel, and most of the local population will turn out to watch. If the PC wins, the fighter's companions will eventually travel to avenge him.
One of the PCs is notified that he has inherited a local brewery, one coveted by a wealthy competitor. A mysterious sealed door is found in the brewery's cellar.
The Coward of the County
One of the PCs is the subject of a lengthy song being performed by bards throughout the area, describing the PC's infinite cowardice. Who commissioned the song, and why?
What are some "adventure shorties" you've used in the past?