Pea puffers! They don't sell for a whole lot but they're always a hit. Try to find a mature pair so you don't have to wait a few months before collecting eggs. They just scatter eggs into a grassy substrate all the live long day. Then you get clever about using turkey basters, small siphons or whatever you want to get the eggs out and hatch the fry in a few small breeder tanks. I'm on my first pair right now and they are almost mature. Got grass growing and everything. It's going to be awesome! I mean, they're puffers! Pure freshwater miniature puffers! A perfect alternative to a betta in a 5g to 10g planted tank in your average office or home. Just gotta get worms established in the sand for them to pick at and a little snail bucket-culture on the side, but those are both easy.
Then there's the the not-often-seen mouthbrooding bettas, betta simplex. Easy to order online and relatively productive.
Bristlenose plecos are also easy and sell well.