What is something really stupid you purchased that turned out far better than expected?
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 while ago someone posted a picture on Reddit of an old cast iron rotary food grater/slicer and asked "what is this thing?". A bunch of people said it was for grating things like cheese or slicing vegetables. Some people posted the original French or Italian names of it, which was difficult to find. Someone said look up "Rotary grater" and they're all over Amazon for dirt cheap. I bought a cheap plastic one for like $20, figuring I'd use it a few times and forget about it.
I use the damn thing multiple times a week for grating blocks of cheese. It can grate a 1 pound block of cheese in like 30 seconds, 2-3 rotations usually gives me more than enough cheese for myself. It's so much easier to use than a box grater, and no possibility of destroying your finger tips or knuckles!