I remember cooping at an IBM site in the 1980's and the senior full time employees would often work on secondary pet projects at work or a few on their side gig

I remember cooping at an IBM site in the 1980's and the senior full time employees would often work on secondary pet projects at work or a few on their side gig, real estate investment/retirement planning. OTOH, one has to remember these employees only recently got freed from the responsibility of starting the work day singing the IBM songs. As a programmer back then, finishing tasks early just meant more features were requested, and tasks added to my queue. Learning things meant reading lots of technical books/papers for work at work or maybe accessing usenet or IRC for cutting edge stuff. If one had access to a unix computer or terminal, lots of text games available for off hours goofing off like Rogue or Adventure. Lengthy compiles/simulation runs might mean a little leisure time reading of the newspaper or a book as you suggest. The only time I have as much idle time as the GP suggests is working in video games and that was only because we were encouraged to play other games during down time to see what the competition was up to and try to stay current with what was available and to get ideas so one is always analyzing the product while playing - i.e. work.