A shavette/straight razor blade: even after deciding on going 100% bald I still love cutting my own stuff, so I decided to go down the rabbit hole of self beard cutting. Never had more fun, outlining my beard has become one of my favourite pastimes ever. And I can have my beard on point whenever I need to, at no cost. Paid ~20€ for the blade holder, and ~8€ for 400 blades. (Bonus addition: hair clipper. Just make sure you buy one that has a lever and enough RPM)
Audiophile IEMs: there's a lot of stuff under the 200€ mark that can make your ears feel previously undiscovered levels of joy. And if you're listening to music a lot of time, this is a damn good investment. Truthear and Moondrop have a lot of interesting affordable options.
Acoustic panels: if you play music on speakers a lot, getting your room to sound drier can really make you discover new levels of detail. Just don't buy the cheapest polyuretane ones, as they only cause random dips in mid freqs.
Smart watch/band: Nothing too fancy, just anything that can notify me of stuff so I can keep my phone away and avoid picking it up to read a simple text message.