Look up your old favourite games. A lot of them have communities that have kept them updated so they’re playable on modern hardware as long as you have a (totally legal) ISO or disc in your possession.
For example, Project Magma has been keeping Myth The Fallen Lords and Myth II: Soulblighter going for 25+ years now.
I loved this game back when Bungie was a Mac-only developer and it holds up pretty well with a couple graphics mods in addition to the work Project Magma has done.
I currently have 4 old Thinkpad T420s (nice) set up just to run this game on Linux with some friends around the kitchen table.
LibreQuake is another one. I’m not as familiar with what they’ve been up to, as I never had a computer that could run Quake in its heyday, so it doesn’t hold the same nostalgia for me. That said, I appreciate their work anyway.
OpenRA keeps Command & Conquer and its various sequels and spin-offs alive, including Dune 2000. Combined Arms is especially fun; a version that includes all the armies from all the games.