Given your parameters, I highly suggest a cat, fish or both. When programming, I've found that my companion of a cat (two tiger striped male and female, 6 yrs and 4 yrs old) are fantastic mood enhancers, that is when they're not crying like babies for food. Barely any maintenance when compared to a dog, but the only downside for me personally is cleaning the litter box (could always get the automatic litter box that…
You could always teach your cats how to defecate in the toilet. (don't teach them how to flush though as they will continually flush to the chagrin of your water bill and mother nature.)
Not all at the same time, I grew up with cats, dogs, iguanas, fish, ferrets, hamsters and rabbits. By far for me cats have been the most worthwhile investment. Dealing with the small inconvenience of the litter box is something I shouldn't complain about since simply having my cats around for companionship has paid off such valuable dividends.