Late to the party, so this advice may not be useful anymore. Someone already mentioned that bettas need tropical temperatures. They really want to be warmer than the temperature of the average office. I've had bad luck getting small tank heaters to keep a consistent temperature. Fish don't appreciate being alternately boiled and chilled. My current Stealth heaters are pretty good, but they're meant for a tank larger than a one gallon vase.
I'd probably get Ghost Shrimp in your situation. They can handle temperatures ranging from 65 up to around 80. Shrimp also aren't the waste factories that fish are; some fish keepers don't bother factoring them into the tank bioload at all. Plus Ghost shrimp are really really cheap; they're usually sold as bait rather than pets. The down side is that they aren't flashy like a betta. You have to appreciate subtle. I personally find it relaxing to watch shrimp. Lots of moving parts.
