If you can afford to get into 3d printing, its extremely useful - probably the most useful tool I have ever purchased. If you do want to get into it and want to learn how to 3d model functional parts, then I would recommend learning parametric modeling with fusion 360 for free.
Programming - people would be really amazed at how useful learning to program can be regardless of your occupation. Python is easy to get started with, and you learn a lot about computer science when learning it as well.
Music production - You can buy a guitar off craigslist for like 50 bucks and just start messing around with it. It might take a couple years, but learning an instrument is a lot of fun. You can also get into music production with free digital audio workstation software such as reaper and some free VST instruments.
Painting - Acrylic paint is cheap so you can start this for a fairly low cost. If you have a little more money, I would say buy oil paint and canvas/an easel, and just follow along with any bob ross video. Oil paint stays wet for days, so its actually extremely forgiving. You can scrape off paint to redo something and never know it was even there in the first place!
It's been awhile since I've used it, but gouache is pretty fun in terms of getting a bit more fluid/rewettable pigments than acrylics, but not as unwieldy and intimidating as watercolor. And also not as smelly as oil paints :)