In 2 gallons, your only real options are shrimp or snails.
Snails are the better bet since it's less likely they'll become a meal for your betta, but shrimp are more entertaining.
You could get a mystery, nerite, ramshorn, etc. Whatever type of snail you want, but if you get nerites or mysteries I'd only get 1 vs a couple of the smaller species.
With shrimp, I'd do a max of 5. Usually they say 10/gallon is a pretty good amount, but since you have a betta in there, you'll want to keep less to make up for the betta's bioload.