Depending on how big your tent is, if you go to walmart or target, you can get a cheap twin sized air mattress for $20. They're actually pretty comfortable. If you're car camping, you don't really have to bring a sleeping bag, you could just wrap a couple comforters around yourself.

If your seats fold down in the back of your car and you have a hatch back or station wagon, you could just put an air mattress in the back of that car.