“ 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 this dungeon, the monsters will not be hostile unless you take something without permission. If you do take something, the monsters inside will consider it stealing and attack you.”
“ A casual remark by one of the party about something met on a previous adventure leads to a summons to and grilling by the Mages' Guild (or equivalent), probably resulting in an expedition back to that area to recover the things for study by the Guild. The party's presence will be 'requested' for this expedition.”