If you really have no time, get a freshwater fish tank.
If you have a little time, especially in the evening or at night, a hamster might be great. I like the Siberian dwarf hamsters. I had one as a kid, and he was really sweet. They are nocturnal, so if you are gone all day your hamster won’t notice. Just give the little guy some attention in the evening, clean the cage as needed (every few days or so), and you can both keep each other happy.
If you have a little more time than that, you might want to consider a rabbit. Ideally, a rabbit shouldn’t be kept in a cage all of the time. It should get to come out and play for a few hours a day and will need a bit of attention. We have a rabbit, and I like him because he’s quiet, he’s litter trained, he’s very friendly. If you want more interaction than a fish or hamster but not the commitment of a dog, a rabbit might be a good pet.
And finally, cats are pretty self-sufficient. Change the litter box, feed the cat, and let it sit on you whenever you are around. Even better, get two cats so they can play with each other.