Earlier this year, we posted a report of a camper conversion kit for the Prius, but it was a permanent alteration that cost USD $40,000. Now a removable tent extension has been developed by Habitents.
Habitents doesn't use any poles and is able to roll up small enough to store in your glove compartment - also something you don't get with a regular tent. It basically hangs off your hatchback with a couple of simple tie-downs, so it sets up easily.
A photo at the Gizmag source shows a view of the interior; the price of USD$90 could be recouped by avoiding one night in a hotel or motel. The manufacturers have a patent pending that would apply to other hatchback vehicles.