If you like schooling fish then 4-5 neon tetras or 4-5 zebra danios are a good choice. Also 4-5 male guppies would make for a colorful and lively tank. Or 3-4 male platies or mollies. The livebearer (guppies, platies, mollies) females are great together as well, it's just that they may be pregnant already and then you'd be overrun with babies and a 10 gal is too small for that many fish!!

Something I'd like to do in the future is an all cory catfish tank. With a 10 gal 3-4 cories would work. They don't just stay on the bottom--many of them swim all around the tank.
A lot of people like to put one male betta in a 10 gal. The bettas love to get out of those little plastic cups they are sold in--they swim all over the tank and really seem to enjoy the space. Only one male though--no other males or females or they will fight.
You could put 3 female bettas together but they should all be put in the tank at the same time so they don't try to dominate each other. Bettas need plants or caves to hide and/or rest in, too. My betta likes to sleep in her cave at night but is always out swimming around during the day.