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 wife got me a fitbit. I resisted a little bit because I didn't want to have yet another device to monitor, charge, and maintain etc. I've been really surprised and impressed and how effective it has been in subtly encouraging me to make some small improvements in my habits. Not a bad deal for $100.