How about community fish?
You can try out guppies, swordtails, mollies, platys, and/or neon tetras. You can put all of those with each other. =]
Pics:
Guppies: http://zoology.okstate.edu/zoo_lrc/biol1114/study_guides/labs/lab14/guppies.jpg
Swordtails: http://www.yourfishtankguru.com/swordtails.jpg
Mollies: http://www.dcfulton.com/fish1_files/mollies.jpg
Platys: http://www.finsandthings.co.uk/platy.JPG
Neon Tetras: http://www.howtokeeptropicalfish.co.uk/neon_tetra.jpg
These kinds in the pics aren't the only kinds of these fish. =] Your pet store will have many more kinds and colors!
Make sure that if you get these fish, you get at least 3 of each, 5 if they're neon tetras.
All of those fish except tetras are livebearers and will probably reproduce like crazy so if you don't want that, get all females or all males. =]
Hope it helps!