A 30 gallon tank isn't big enough for 2 rats? Seems like it would be plenty big enough, and rats are hands-down the best rodent pet. I had 2 in college (I think I kept them in a 10 gallon tank), and they were awesome. So social, cuddly, smart, and great for freaking out my rodent-phobic stempmom!
I also had a gerbil, but that thing was so mean I had to wear leather gloves when handling it, which I only did when absolutely necessary. I didn't have him for very long.
Hamsters are also great...much cuddlier and funnier than mice or gerbils.
Mice are cute, but I hear they tend to be bitey, and they're so tiny and fragile I'd always be afraid of hurting/losing one.