A wizard has been living in the city for years, but now he plans on duing some interesting new research so he recently bought a nearby tower & is planing on moving there soon.
The party is contacted by a servant of the wizard who tells them that his master has heard of their abilities & wants them to clean out a nearby tower & dungeon (the afore mentioned area). While the wizard could do so himself, he is very busy with his research & is sure the party is more then capable of the task at hand.
When they reach the tower/dungeon they find it infested with all kinds of extremly rare monsters & many traps.
The twist :
As it happens the wizard actually wants the players to CLEAN the tower. The monsters were brought in for his research & the traps are designed to keep them seperate from each other. When the party returns to collect their fee they will find a very angry (& possibly powerful) wizard.
This should allow you to place a varity of rare creatures at one place.
It might be a good Idea to point out to the players that these are very rare cretures, but good PC’s should notice that the traps are built in conjucture with the nearby creatures (design traps so that it makes sense that they are there to keep the cretures in).
P.S : I actually put a broom & mop near the enterence (my Pc’s were pretty thickheaded) but I think it would be too much of a give away for most players.