Any art medium that requires canvases. A small one at WalMart is no problem, but as you move to larger works and need better canvas material it gets expensive fast.
Oh! And Arduino programming. A simple kit to get going isn’t too bad. Then you’re trolling Adafruit for parts, then you go big and start importing from China directly. Now you’re building a garage addition for the electronics lab… Or is that just me? At least it’s also able house my first motorcycle… First…