“ What if you send the players in a dungeon that was already raided? Maybe you could have traps that are already set off, or the corpses of monsters etc.”
“ In the middle of an unimportant combat with some bandits a burst of wild magic transforms all of the PCs and their opponents in to random animals and monsters. They retain their intelligence (though not, of course, the ability of speech). They can either carry on the combat in their new forms, panic, or otherwise react how they see best. After about an hour, they return, unharmed, to their normal form.”
“ A city is being plagued by a swarm of winged mice. The vermin can get anywhere, and they can easily avoid their normal predators by simply flying away from them.”