There are a few things with a Prius that if I had would help living in it. The one on the top of my list is an option.
That option would be a sun roof, aka roof vent.
On the vandweller (cheaprvliving.com) forum there is a lot of talk about venting your campervan and if you do vent, what method is best for stealth.
I generally like it warmer than not so it needs to be really warm for me to like a roof vent. With record warm temperatures (80-90) in Phoenix this week, a roof vent would be great right about now.
Just cracking the sun roof and cracking the door windows under the rain guards will keep the interior of the vehicle a lot cooler in hot weather.
This would be a great help with the amount of ice someone goes through with an ice chest, or how well my compressor fridge works. My compressor fridge not only has to overcome the heat getting through the walls of the cooler but also has to work harder to dissipate the heat from the coils.
Cracking the roof to disapate the heat is especially helpful if you find a place in the shade. If you are in direct sun and your vehicle is a greenhouse then there is not much to help you, unless you set the Prius up as an air conditioner while you are away.
See post on using the Prius as an air conditioner.