There are any number of different types of danios- longfin varieties, albino varieties, etc. . . I really like cherry barbs- I have a few of them in my 20 gallon community tank, and they look awesome after a couple weeks of getting used to your tank- their colors really pop. You can look at rasboras as well- pork chop & harlequin rasboras are really pretty. My pork chops school with my cherry barbs & add bright orange flashes of color to the mix. And people will scoff at you for it, but there are always guppies. Look into them a little and you'll find that there are some REALLY pretty ones out there. One or two of those in addition to some danios or something else on the "plain" side can look really cool.
I work in a pet store and we sell TONS of 2.5 and 5.5 gallon aquariums. It takes longer to cycle a small tank for a lot of people (many crash it by adding too many fish too fast & have to start over), but it works. We get customers in who keep 10+ fish in their 5.5s. The trick is to keep up on maintenance. I also recommend 50%+ water changes every week, but on a tank that small, you're talking 2.5 gallons.