The fun part was that they turned off the timesharing service at night - but they didn't want to power down the Sigma 5 for fear that it might not start back up in the morning. There were occasional overnight batch jobs to run, but mostly I didn't have much to do.
I already knew BASIC, having punched programs on paper tape in high school to run on Transdata's service (which was how we got acquainted). I found a copy of the Algol-60 report at the office and thought it looked interesting, so I read it, tried out a bunch of programs, and learned Algol.
Then I found an assembly language and opcode reference for the Sigma 5, which was fascinating. There were plenty of blank cards to punch, so I learned machine language too.
I could have just sat back and done the night operator job and not much else, but man, there were such interesting things to fill in the rest of that time. And it's stayed interesting ever since.
Of course that was an unusual situation, and I suppose not one you could repeat now. After all, how often do you get a chance to have a whole computer all to yourself?