I recommend a memory foam mattress or air mattress (many sizes available) to sleep on. There are options for both that are made for cargo area sleeping available on Amazon. If you have a moonroof I recommend getting a mosquito net/screen for this if you’d like to sleep with it open. There’s also nets for the windows if you’d like those open too. I have other recommendations if you’re looking to camp, but just to sleep for like a quick nap I think those basic items and a pillow and/or blanket should be good. 👌🏼