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.
Bed sheet suspenders. Dumb problem, stupidly cheap, horribly made, and ABSOLUTELY fixed the friggin sheets being yanked off the corner of the bed twice a night by my tumble-dry-medium sleeper of a spouse.
When they finally broke after almost 2 years I sewed some that'll last 10 years and I don't regret them at all.