To go along with the mattress theme, and since I’ve had two back surgeries (in my 30s after a car wreck), my sleep number mattress was a big expense but I was miserable and could never get a good night’s sleep before it. It’s been a game changer.
The other is a little more niche, but I’m a runner/endurance sports and my Balega socks have been incredible. They are $14 a pair (yes, $7 a sock), but they are easily worth every single penny. I’ve done 30 hour hiking/running events and my feet were the only thing still dry on my body thanks to those guys.