Originally Posted by TMaier
Betta or opal gourami.
I'd be more inclined to go with the betta for a 29; opalines get to about 5", for my tastes a little big for this size tank.
In either case, the OP will have to keep an eye out for aggression. Bettas can be easygoing tankmates, but they can also turn out to be little serial killers. And gouramis of that species can be aggressive, too: I had a gold gourami once that was a genuine butthead.
Honey Dwarf gouramis are smaller, but they have some lovely color variants now, and they're quite peaceful.