Group of swordtails? Couple of twig catfish or bristlenose plecos? Group of smaller rainbow fish?
+1 on the group of swordtails. I would go with 2 males and 4 females and you could always mix up the colors with no compatibility issues. I was in the same boat as you a few weeks ago and the swords are a nice additional to offset size. Along with that, I picked up a pair of German blue rams that add great color contrast and make a great centerpiece fish.