Tent carpet, good airbeds and sleeping bags, a folding loo for night-time and nice reliable and comfortable chairs. I ought to add "space" too, especially for longer stays so you can spread out....
------------- Love our set-up and need no more tents or gear, so trying to stop looking!
I know you wanted lights/furniture, but I've got nothing special in that department. My car's full enough as it is, & I'm a solo camper with a large saloon car! (& only basic kit, just lots of it!)
I agree that it doesn't need to be colour matching to be comfy - and not even tidy. When I travel solo, I have space in the car for all the colour matching luxury things, and the tent looks like this:
... but when I travel with the kids, it is a completely different thing - still comfy, but looks more like home...
------------- Proud owner of a 1987 Sprite Alpine 370 EK, a cheap popup tent and a beloved retro Trio frame tent from the early seventies, called Giraffen.