Bettas are my favorite. A big snail will liven up the tank, and you could even get some Cory cats if you had some coverage.
They have really cute logs at petsmart and Cory's would love to go through it, and theyre always at the bottom and relatively quick.
Get sinking food for them and you'll see them snatch it up. Plus they're super cute!
If you're looking for something more mid level or top water fish, go for the little danios. They have black stripes down them. Pretty bland fish but you can get just about a school of them and theyll be happy. He might chase them some but they'll mostly keep to themselves.
I recommend you feed them on separate sides of the tank.
Shrimp are also really neat, but sadly, a lot of times a Betta will make them a treat. If you had a lot of java moss then they'd be able to hide well, but theyre really cute and neat if he doesn't mess with them.