First I would throw in a couple more neons. Neon tetras are schooling fish, which means they prefer to be in groups of three or more. Adding only one more will make them less stressed, and it will probably make their colors brighter. As far as other fish to put in there, you can do other kinds of tetras such as cardinal tetras, black skirts, red minors, or others. Remember that they will also be schoolers though, so you'll want to get them in threes.
Another good choice is livebearers. These include mollies, platies, or guppies. These guys are best bought in at least pairs, but beware that they can produce babies very rapidly, so make sure when you buy them that you ask for one sex or the other if you don't want a tank full of little ones. It's very easy to tell the difference just by sight, so any good pet store employee should be able to get you what you want. Occasionally mollies can get a little aggressive, but since neons swim so much faster than they do, you won't really have to worry about the mollies nipping their fins. Buying female mollies will take care of that though.
You can also add a cory catfish as a bottom feeder to eat excess food and other debris. A few mystery snails or a small group of otocinclus (they're schoolers too) will help keep algae at bay and round out your tank.
Good luck.
