zebra danios are beautiful and graceful fish they usually retail for about .99 to 1.99 a fish..
u could also look into maybe 2 sunburst gouramis.. they are beautiful lot of personality a little on the larger side they get to be about two inches maybe max. and then you could have a school of danios.
I'll only tell you what NOT to get:
Livebearers (guppies, mollies, platys, etc). Will have up to 80 fry every 4 weeks (each female). If you want any of these, get males only. Chance are extremely high that any female you will purchase will already be pregnant. They breed like crazy (then don't forget in a few short months the fry will be reproducing as well).
Common plecostomus- (plecos or plecs for short) no matter what the peeps at the pet store will tell you, they need a MINIMUM 55 gallon tank and that's pushing it. Any tank smaller will stunt them (they'll quit growing externally, but the organs inside will keep growing, causing it to eventually die) They naturally grow to 2'.
Iridescent sharks-same thing-they grow to 4' and in my experience they are not real slow growers. Could start them out in a 55 byt it won't take long for them to need a 200 gallon, then 400 gallon, and up (you get the picture)
A tank too small for them will not only stunt them but will smother them as well.
It's a matter of what YOU choose. Just do the research on the web before you purchase them so you can make smart choices and not have the problems a lot of us have suffered. We learned the hard way.