I'd say guinea pig as well I got Pepsi when I was nine [which means he's about 7 years old right now] and I took care of him all by myself

I have found guinea pigs to be much more affectionate than hamsters, and if you are careful with supervision they can actually interact quite well with dogs and cats [my one cat is VERY protective of Pepsi, and the dog/other cats tolerate him]. They do require a bit more care than some of the other small mammals, but I think it's worth it in the end. Plus, they're quite happy to chill in their cage or on your lap, and they're so darn cute!

One thing I would say though...guinea pigs are quite social so if you are considering a piggy, you might want to think about getting two.