in a tank that small you are limited to what else you can add. i would personally go with an apple snail as a tank mate and no other fishes, since male bettas can be a lil snarky with other fish (this does depend on the betta's individual temperment, of course). One thing to keep in mind with apple snails is that sometimes fish will confuse their tentacles for worms, so you have to watch for mr betta nipping at them. i've kept three different male bettas with three different snails and i've never had problems, but that is most likely due to the fact that i kept my tanks well fed and my bettas were really mellow.
if you do decide to go with other fish, stay away from any with long fins and/or vivid colours, since the betta may think "another male betta? i must fight for my territory!".
you could add a small group of some of the smaller tetras, but i wouldn't put more than a few in with the betta, since you are working with just 10 gallons here. and you need to make sure that your filtration can handle the extra bioload that more fish will create.
if in a few months you find yourself looking at your living room/kitchen/fish tank area, and you notice that the tanks seem to be growing by number...... congrats! you have MTS! (multiple tank syndrome). hahahaha! trust me.... it happens!