They're evil, right? So they could go on a mission to kill a litter of kittens. Maybe the king's kittens, or something. This (potentially) could work really well if one or two of the PC's takes pity on the kittens if, despite being evil, they still have some sense of cameraderie, and they won't just kill each other over it.

Write offensive graffitti on the side of a good temple or some such place.

Moon the head paladin of Pelor (or whatever) while he's making his speech to the masses on some holiday.

Well, you get the idea. This sort of thing seems more funny than the more obviously serious hacking/slashing/burning/pillaging angle. The essential idea is that the main evil force (their boss?) is really bad at it.