I play with autonomy on unless the Sim has to read a book, then I turn it off so that they'll sit or stand still long enough to finish it. Otherwise, my Sims are usually pretty good at doing what they do when they have down-time.
When autonomy is on they do various skill-related things like request a pen-pal, practice writing, practice writing jokes, hack something, mod a game, make a virus, practice programming, play chess, paint, play guitar, research plants, or inspect their garden. Some practice yoga or meditate on their own. I've had teens hack their own grades too. They generally don't run out of things to do. I can tell when they're getting bored when they start watching television, looking at their phone a lot, dancing, or chatting online. That's my cue to have them work on something. Once in a great while, I run into a Sim that doesn't do any of these types of activities and it drives me nuts.
I recently started playing a young adult male Vampire that I found in a save file that I downloaded several months ago. I mostly wanted to use him to tour the save so I could see how it was set up. Right away I noticed that he cycled through things like watching television, browsing a book, or chatting online. I have yet to see him do anything on his own in connection with his garden or using a computer for anything but playing video games and chatting. I turned autonomy off for him when he started canceling work-related tasks. This type of Sim is one that I'm not used to dealing with. He's on the verge of being kicked out of his fancy house and relegated to townie status.