Other small pet?

Hi, I have two very sweet guinea pigs and I absolutely love them. I'm thinking about adopting another type of animal (what can i say, they make me happy) and my landlord company doesn't allow cats/dogs, just animals in contained cages(they don't know about floor time, hehe). I've been researching everything--mice, hamsters, gerbils, degus, hedgehogs, rats, chinchillas (though I'm not sure I'll have space to accomodate my piggies AND chins) and I've even thought about reptiles like geckos or turtles/tortoises. (No snakes though). I was just wondering if anyone had any opinions on the temperment or care of these animals. I'm looking for a relatively low-to-medium-maintenance animal, that doesn't require an enormous living space (something that could sit on a desk or table would be ideal, as I'm running low on floor space after the piggie cage). There's also a good chance the animal would live in the bedroom, so animals that are super-active at night might not work. I'm not necessarily looking for something that likes to cuddle/be held--it'd be nice, but if not, they should at least be kind of fun to watch.

I'm a college student so I don't have a huge budget, which is why hedgehogs and chins might not happen. I'm thinking gerbils or hamsters, maybe degus or rats if I can find the space--do any of these animals have cages that can go up, rather than out?

Thanks for any info!