Most Gobies are a great avenue to look down, they are usually inexpensive, small, and very colorful. They also have great personalities. As with any fish you will be purchasing down the road be sure to get it/them from a repsectable business/vendor, be sure it is eating prepared foods, or if possible ask to see it fed. Many times new animals will starve because they are not properly weaned from food that occurs in nature to prepared foods.
Stay away from any Tangs, Angels, Anglers, Anthias, Batfish, Boxfish Butterflyfish, Damsels, Dragonets, Eels, Groupers, Foxface fish, Filefish, Hogfish, Lionfish, Pipes, Most Dottybacks, Puffers, Seahorses, Triggers, Most Wrasses.
You may also want to consider looking into Pseudochromis, Chromis, Gobies, Darfish, Blennies, and some Basslets.
Or my favorite nano fish, but I must warn you, very picky eater, Opossum Wrasse.