Anyone familiar with Greek myth and legend will know that Daedalus (father of Icarus) was the architect who designed and built the Cretan Labyrinth for Minos. He and his son were locked in the labyrinth after their work was complete, and escaped by making wings from wax and feathers.
The Despotic Queen of Shiant has decreed that a competition will take place in the month of Oconerry. She will have built a labyrinth through which the contenders must pass in order to win the ultimate prize: her hand in marriage. She needs a maze so cryptic and impossible to solve that she can be sure of her prospective husband’s abilities. So she has drafted in the best architects in the land to design it for her: namely the PCs.
You give the PCs a budget, a copy of the Monstrous Compendium/Manual or other such bestiary plus a list of prices for stone, wood and materials, and they can set to work designing their own dungeon. If they think the prices are too high, they could open up their own quarry (at the protest of the locals) or barter with merchants (lots of scope for roleplaying) and in order to acquire the relevant beasties and monsters they would have to go on long expeditions to very exotic and dangerous locations.
When the work is completed, the PCs are incarcerated in their own dungeon by the evil Queen (who is loth to pay up) and they must escape (needless to say all the codes the PCs set have been changed, all the traps changed in subtle ways so they are hard to disarm).
I like this idea because it will give the PCs food for their imagination, an idea of how hard it is to be a dungeon-designing GM and last but not least a shock.
It also involves the GM in very little work compared with the usual hours of preparation.