Can't recommend KSP enough!
I had never played a sandbox game (although now playing in career mode).
This game takes game/simulation into the realm of hobbyist/enthusiast. I've now had a crash-course (pardon pun) in astro-navigation in Newtonian physics.
You will want to install the Scott Manley list of mods
to get the full effect- which is beautiful btw.
The community is active, the developers commited and it looks like the push is for deep space.
I also still play Enemy Territory: Wolfenstein, a game now over ten years old(!) still great FPS fun.