Also be sure to get the free epic store game of the week each week. They are sometimes really expensive big games. I have so many on my account from over the years. Didn't pay anything.

bitcoingecko

On Linux try OpenTyrian, OpenTTD as two fun oldschool games. Otherwise you can install RetroArch and play any older game across any console. 

nout

gaming

I don't use Windows, but my daughter does (has to for her job), and this has some great games, apparently. It's a lovingly-thorough spreadsheet with links to over 200 games on Steam and other places for free. A handful run on Linux or Mac, but the creator didn't note that, so this is probably most useful for Windows folks. I am slowly going through it to see which ones might be playing on my Linux systems.