Well, guppies and endlers breed like mad, even if you don't want them to. That said, I don't recommend that unless you are breeding a very high-value variety because you're going to have trouble off-loading the fry if they are common ones.
Cories are a pretty easy egg-laying species, and you could fit a nice breeding group of dwarf cories like pygmaus or hasbrosus in a tank that size, or a slightly smaller number of small regular cories like pandas. Cories also aren't prone to preying on their own fry, so you can leave the parents in if you don't want a separate rearing tank. I fed mine similarly to the baby cichlids below, except I just put the food on the bottom of the tank and had a snail in there to help clean up leftovers.
A pair of dwarf cichlids like apistos, rams or kribs could breed in that size tank. They care for the young, making your life easier and increasing survival of the brood, but you'll need to remove either parents or offspring once the fry are large enough to care for themselves. I fed my baby cichlids by putting a few large miramo balls in the vicinity of the nest, and once fry were free-swimming I used an eye dropper to put pre-soaked hikari "first bites" on the top of the miramo in a very thin layer. I'm still not entirely sure if the fry actually ate the first bites or if it just fed an infusoria culture on the miramo surface, but it worked pretty well regardless. Once they got a little bigger, I started adding a little frozen BBS to the mix until they were entirely on frozen BBS.
You could maintain a small breeding colony of sparkling gouramis, but it would need to be pretty heavily planted. I haven't bred those successfully myself (tried, but had to downsize the tank before I could get any babies).
If you set up a spawning grate or something, you could try breeding egg-scatterers like tetras, danios or rasboras. The parents can't stay in the tank though, as they will eat any fry (and any eggs they can get too). Fry need pretty specific foods though, like infusoria.
Of course, you could also breed a whole lot of neocardenia shrimp in there instead.
