As long as you have your waterline dropped 1-2 inches from the top of the tank then you should be relatively fine with almost any fish without a lid.
You could do 1-2 medium fish like a gourami or betta as a centerpiece fish and then have a small school of nano fish like neon tetras, pygmy cories, or a few danios. You could also do 3-5 livebearers (depending on size) like guppies or platies or mix them with the above combinations.