What do you do at work, when you have nothing to do?

My work can be quite busy (system admin / devops) but sometimes I have nothing to do for like 3-4 hours. I tried to learn some new things in this time (Go, Lua or just some more about Linux) but many times it does not even feel good. I feel much better when I learn something for some purpose.

Also I cannot leave office in this time to go to gym or go for a walk because of COVID and also because it's not time efficient. Plus I can still get call anytime that there are urgent things to fix on a server XYZ.

So what do you do, when you have such free time, but you don't exactly want to spend it watching stupid stuff on YouTube?