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.
A toy accordion I bought at a truck stop 30 years ago. I blew all of my $30 in vacation spending money on it and everyone said I’d regret it. It ended up kind of joke\prop instrument in all my bands and I still have it and it’s still fun to play.