Neons can be nippy, so I've heard that they aren't the best for bettas. But they may work.
You want something that won't nip the betta's fins and harrass him, but that doesn't look like a betta. (bright colors, and long fins are a no-no. Thus fancy guppies are out.)
Cories can be great for betta tanks. In a ten gallon, you don't necessarily need any more top dwellers, and a small school of playful cories liven things up. (i don't remember which cory species stay small enough for a 10 gallon, but they exist.)