I’ll start by plugging Harvard’s free courses catalog as well as Udemy
Edit: Gonna add 2 more I remembered-
Blender - I wish I had more time to learn it, but I did start the infamous “Donut Tutorial” once!
Watch Cartoons Online - Lots of good older stuff!
Maybe this is sorta dumb, but meditation is a free way to feel good and spend time, and also a free method of stress relief and to reduce suffering.
It’s not free in terms of your time & energy, and it might cost some money to learn, but the best meditation manual I know of is free online, or at least it used to be - it looks like it was locked down on archive.org (where it used to be freely available), but you can still find it on Anna’s Archive, and you can probably find it at your local library. Either way, you can learn to meditate for free, that’s how I did it.
Running is likewise relatively free (you do generally have to pay for running shoes, and athletic clothing can be expensive, but it’s relatively cheap over the lifetime of those items, and it’s cheaper than most other activities). A great and accessible way to feel good and stay healthy.
Plug for Medito, which is a free non-profit for guided meditations. Not sure if the apps are open-source but still, they’re a good organization in stark contrast to the many other meditation companies that are very much for-profit.