Ghost shrimp are pretty fun to watch once you find them - it's neat how they eat :D I had three a while ago and one vanished and I've had two ever since. I thought they would be snacks as well but I'm starting to think that I'm the one who killed the third accidentally in a WC ^^;; My bettas do the food scavenge thing with them.
I'd avoid ottos unless you plan on a live very planted tank that's producing algae and you're also growing algae for them on the side. They enjoy hobbies such as starving themselves to death by not eating wafers and ignoring fresh cut veggies because they are all wild caught and don't get that it's food. :/ If you have the right environment for them though they are super little guys.
I have some  kuhli loaches in a sand bottom 10 which there is a lot of debate on if they can be in a 10 or need at least a 20 but they have fattened up and gotten great colors since I got them so they seem to be doing well. They share their space with a young female betta.
I've had good success with Platies as well even though I was hesitant to put something in the same water level as my betta. [Try for all females :D] And as many people have said Cories are darlings :D If you can I'd recommend shipping a few pygmies in though just so they could have a larger school. Other then that Petco carries Juli Cories that I find to be super adorbz.