A commoner has run into a LOT of money! Unfortunately, everyone knows it. Good luck getting out of town in one piece!
The local lord has been deprived of his prize family heirloom, the thief isn’t exactly a thief, but rather an absent minded wizard. Get the heirloom from the kindly but absent minded professor. Simple, right? Well, except for the fine print.
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.