How much cover does your tank have, and is it cycled?
If it has a good ammount of cover (decor, plants, etc) and is properly cycled, then you might want to consider a school of fish. You could easily fit a school of 6 or more harlequine rasboras in there. They are very active (if just a little bit timid) and while they might not be as flashy as the other tetras, they do add a lot of life, and their rosy reddish color contrasted with the black mark is really nice :) If not the harlequine rasboras, then a school of cardinal tetras or neons would be good. I personally have no experience with either the cardinals or the neons but like most tetras they might get nippy if bored or if their schools are too small.
If you decide against the rasboras or tetras, then you could also fit in either a few more platies or a group or two of four or five cory cats. Cory cats are great and come in all different shapes, colors, and sizes, but in general they prefer to group in a same species group. They need cover as well, and you have to be careful with their barbles. If your gravel is too rough it could scrape and hurt their barbles. The cories would need sinking carnivore catfish tablets and maybe the occasional herbivore tablet for them to sample. :)
Last edited by JKfish; 01-11-2012 at 08:24 PM.