The PCs are stuck in a town with a strict peace policy, the tiniest scuffle can land you years in prison. The town also happens to be a tourist haven, so inn-prices have skyrocketed. The only way the PCs can rest is if they splurge on a room, with their enemy.
The spellbook of a long dead wizard that has gained the ability to read itself and cast spells from itself. If could be amazingly powerful as an ally, or use its magic to manipulate and rule a kingdom.