What do y'all sleep on in your Prius when camping?

I've been happy with a Hinman. (Mine is several years old and not identical with the current models.)

 

I got a simple foam roll at an army surplus store, it's thin and stiff. (similar item at target) Then I got another one at Gander Mountain or something, it's thicker and softer. Either one by itself wouldn't be too comfy, but the two stacked are just perfect, and still cheaper than even one self-inflating pad. I put a sleeping bag atop them, and lay a pillow on the center armrest. I sleep with my toes at the tailgate and my head up between the front seats on the armrest. It's good for one tall person, not so accommodating of two!

 

I use a ThermaRest.
They come in many different shapes, sizes, and comfort levels. They store compactly, and fits in the Prius "basement" with room to spare. They're easy to inflate and easy to repair.
As a bonus, it can be converted into a chair if you buy the frame.