A list of suggestions based on past successes:
A small pack of sparkling gouramis
One pea pufferfish
4-5
ember tetras, chili rasboras, or other equivalently tiny shoaling fish
1 betta, completely alone if he is especially sassy
species-only shrimp tank of a rare color or variety
1 scarlet badis, possibly along with other tiny peaceful fish
Temporary grow-out and observation tank for new baby kuhli
loaches
Clown killis
I VERY highly recommend keeping live plants and cherry shrimp with almost all these options. While you might have the luck to get an especially murderous fish, in my experience, shrimp can survive and successfully reproduce while sharing a tank with pea puffer and bettas. You won't get a massive colony but they won't die out.
Aquatic Arts is a great source. It's pricey, but the quality's worth it. I highly recommend waiting for a sale though.