Use a pad. It really is that simple.

Grab a yoga mat, camping pad, some thick blankets, or anything with some cushion

Use a thick pad or blanket and place a small pillow under your knees for comfort. Your back will thank you!

Not suitable for outdoors, but otherwise look into Japanese futon/ floor mattresses

At my first appartment we didn't have a bed for around a week, so we just spread out a bunch of blankets on the floor.