Ferrets are playful and affectionate. Very cute to watch, just look up a "weasel war dance" and tell me that's not adorable!
Their litterbox smells worse than a cat's though (I'd say even worse that a full size pig's pen x.x).
Guinea pigs are good pets, they recognize their owners and "talk" to you, but not really too playful in the trick and jumping around area.
But as a plus, they don't smell and are relatively cheap easy keepers.
As mentioned, no matter how small the dog, it will need a daily walk - not just to go poo but for it's mental well-being.
I find cats very affectionate to their owners, most are more stand-offish to strangers, but there are exceptions. And you can totally teach cat's tricks too. I prefer cats over most dogs.
I have dogs, ferrets, and a guinea pig and I've owned cats, birds, rats, fish, snakes, and hamsters (would never recommend hamsters or gerbils or most mice as a pet, unless you're very enamored of the little monsters), among others, in the past.