when it comes right down to it, guppys, bettas, neon tetras, and malawi cichlids (not sure if a 29g is suitible) are the most colorful fish that you dont have to get shipped from far away places and spend a lot of $ on.
then you have stiphodon gobies, other species of gobies and gudgeons, exotic livebearers, quite a few species of cichlids, gouramis, exotic anabatoids like ctenopomas, and some species of US native darters, like rainbow and orangethroat.