Depends on the hobby because I have too many...
Photography - not much unless I buy used, which is what I usually do. So some funky old lenses and adapters to get on the body, which usually nets some fun results.
Data hoarding - a couple of hard drives. Which I should really do, I'm coming close to 5% free (which sounds like less than it is when you've got about 115tb, which is why I'm not rushing out).
Computer Vision / AI - more processing power! But I make work buy me that stuff so its fine.
General compute - about 5 more tiny/mini/micros at the usual price (plus the usual upgrades) to either ads to the cluster or make a new one! Or one or two of the newer little beasts that I will probably be waiting another year or two to pick up.
Bicycling - a good bike for getting around! My bike is about 15 years old now, it was new old stock because no one liked the color apparently - which made it about 1/2 the price of what it should have been. I've been replacing gear on it over the years, but I'm tempted to buy something new, honestly less for going fast and maybe more upright, like a hybrid. Which I can get an OK model at that price.