PC games that can be played in short bursts?

I spend a weird amount of time waiting around for people online, which means I spend a lot of time going 5-30 minutes kinda just sitting around not being able to do anything. What are some good PC games I could boot up to play during this time? Nothing multiplayer - it has to be something that I can save and quit out of at any time. I'd also like it to not be story based, if that's possible (I like to immerse myself for long periods with story based games). Paradox games like Crusader Kings II, Europa Universalis IV and Stellaris are good examples of the sort of thing I'm looking for.