Walking mat for said standing desk. A coworker of mine swears by this, I'm still considering one. They even walk while watching movies on the TV :D
A proper working chair, your ass is going to be on it for 8+ hours. Don't skimp on it.
Steam Deck. Seriously. It made gaming fun again. My gaming PC hasn't been booted since September when I got the deck. I can only do one thing with it, play games. It's always suspended on exactly where I left with the game I'm currently playing through. I can use it on the couch, toilet, train, vacation or even when there is downtime at work =)
It forced me into the gym regularly for months, I didn’t have to think about what I’d be working out, and my trainer kept me honest, kickstarting a habit and confidence I’ve tried countless times to develop.
Of course, YMMV and some trainers are terrible, but I would have happily spent much more.
Similarly, a membership to a gym or other kind of physical activity (martial arts, etc.) that you’ll stick with is worth it to anyone who largely sits in front of a computer for work.
Pretty much anyone can also benefit from a few months of therapy. We all have baggage from our childhoods that can be overcome, and all of your personal and professional relationships will prosper as a result.
Good standing desk - $500
Adjustable dual arm mount for monitors - $100
RTX 3090 to play with Stable Diffusion - $1000
Good reverse osmosis filter - $400