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.
My bathroom has the worst-designed sink ever, with a tap that's too long, a sink basin that's too thin, and a medicine cabinet that means that if I want my beard hairs to fall into the sink while trimming I have to literally have my face pressed up against the mirror which is, of course, not something you can do while trimming a beard. I got one of these things too and it's so good. 10/10, would buy again.