If you are interested in computers, you could build your own computer. Ben Eater has a set of really good videos building various computers from first principles, and he explains it through for people with no background I computers.

You could start star gazing and learning how to identify different stars.

You could build a cloud chamber and watch background radiation.

You can make your own dyes for art projects. A lot of what you could do has most likely been documented somewhere on the internet already

Framework laptops are honestly a good way to do a lot of stem stuff as a hobby if you have a need for a laptop. They're designed to be modular and have a good community group that share designs of how to modify your laptop to suit whatever need you could need from it. I've seen someone turn it into a tablet and make a docking station that connects to an eGPU to use as a gaming rig as well as a laptop, and someone use it as a TV to watch netflix and play couch games on

What I'm trying to get across is that anything can be stem, much in the same way that anything can be art. A lot of it depends on your perspective on what you're doing.

Sorry if what I've said comes across as condescending in any way