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 needed a "lap desk" or something to put my laptop on, but I wanted it to be low-profile and I could only find a wooden cutting board. Now wooden cutting boards are the only thing I use as lap desks because most actual lap desks I find are super bulky.