Rainbowfish awesome! Definitely be a good choice, and prettier (and more expensive) than danios. A large group of serpae or red minor tetras would also be great, and they add some permanent color to the aquarium. They aren't really a schooling fish, but they interact a lot with each other.
For some medium sized catfish, I'd go with bristlenoses. They get fairly big, are active during the day, are fun to feed (zucchini, vegetables), eat algae, and are so easy to breed. Males guard the fry. They will nibble on plants if they are hungry, but with fast growth rates, I don't notice it.
A dwarf cichlid would be cool. A dozen kribs? I've got some Apistograma cacuatoides that are also very cool.
Best of luck!