I bought a piece of 1.5 inch stiff foam to try to fix a sag in a bed. It didn’t work but having that thick piece of solid foam around has been a life saver.
Need something flat to put a laptop on? Throw it on the foam. Going to be doing something that requires you to be on your knees for a while? Get the foam!
It went from stupid purchase to something I’d gladly replace if it broke.
I picked one up a few years ago at Ikea, not the cheap plastic one they sell, but the super long steel one. Putting on shoes is like sliding into a comfy bed. My wife has been ridiculing me since, until recently when she tried it out.