You could always try shrimp, but many, many bettas just see shrimp as a nice little treat so personally, I wouldn't try it. I like my pet shrimp too much
You could try neon tetras I guess, but I think that were I to get neon tetras, I wouldn't want them in anything less than say a 20 gallon long, but that's just me. They are a shoaling fish and I would want to put them in a group of at least maybe 10-12 in a long tank. However, this is just me. I am sure that they would do just fine in smaller groups in a ten gallon
So you could always try a small group of those.
A really neat fish you could try is the pygmy cory. They prefer to be in groups of 6+, or so I have heard, but my 3 pygmies seem content in their small group. You do, however, need sand for them. Not sure they would do so well with gravel.
Hmm, what else..It really just depends on the character of your betta. I have seen some really docile bettas with brightly colored, long-finned, active fish, and then I have seen some that will go for anything in sight.
Most of all, just watch out for the nippy species. There are some fish that will actually go after the betta. Some species of tetras, I believe.
If nothing else, you could always go for snails
Good luck! Hope this helped!