I'll second the Endler males. I've got a trio of males and they are great to watch, very busy and colorful. I also love the bettas, and not just the long finned ones. I've got some betta imbellis and they are great to watch, lots of personality and the males are beautiful. You might not like the females, they're more drab.
How nano are you going? I hear German Blue Rams come up a lot if you're not getting really tiny. I recently got some Bolivian rams and love them, but they're in the 40g community.
I have a very active halfmoon beta. He really is the most awesome fish, very interactive and always excited to see people. He also will jump to get food off your finger if you hold it just above the water's surface.
I kept a trio of dwarf puffers in a 15 gallon. I wouldn't say they were more trouble than any other fish. Just keep up on your water changes. Personality wise they can be a problem though. They are species only, you might get shrimp or otos in there but mine would attack both so YMMV. I eventually had to split them up and give them away because my one male decided he didn't want to share the tank any longer.
edit: That fluval spec is pretty small, you might get one in there but any more and they will likely kill each other. They are mean little fish.
Yeah I'd read about them being species only but also read that there shouldn't really be any issues with otos. I'll maybe have a good think about it and see about sexing them so I only get one male. Thanks for the advice
Betta sorority: color + personality + never a dull moment.
They come on sale at PetSmart occasionally for $0.99/ea and PetCo will match the price.
Betta sororities are amazing and rewarding - but they require dedication and a close eye, especially early on while the females sort out pecking order. And there's always the chance that you'll run into a female that just can't be housed with others.
I love 'em, though. I've actually just dirted and filled an old tank to start a new sorority. Just needs to be planted and cycled.
Danios need a lot of swim space horizontally. I got a few in my 10gallon, and I felt so guilty about their space when I saw how active they wanted to be. I'm getting them into a 20Long as soon as my roommate lets me put a tank in the livingroom.
Though, if you're in the market for a long tank, with lots of swim room, by all means, go for the zebra danios They're beautiful, almost iridescent with deep blue and light gold, and they're so active!
"Aquariums are like science, art, and hypno-therapy, all rolled into one," I insisted.
"You're not putting a hundred gallon tank in the living room," my roommate replied.
If you're looking for a group of schooling fish with more personality, I really like my spotted pygmy Rasboras (Boraras maculatus). They are very playful and interact as a group. Not very interactive with me, although they will come over to the corner of the tank to see me when I'm ask the desk when it's feeding time. Good with other community fish. Had them for a while with pygmy cories and they'd school together and play in the water current.