I'd vote for a guinea pig, as they can be happier in a confined space than a rabbit would, and both dogs and cats tend to see bunnies as prey. A guinea pig can be in its cage, and can come out for playtime in a ball to run, or fence off an area for them to run and play ... they are sturdier than a hamster, and easier to interact with than a gerbil. A parakeet's noises might prove irresistible for the cats, and one cannot underestimate the persistence of a curious feline.