I vote pico reef! I recently set up a 5.5g with 8lbs of LR, some coral frags and a rippled clown goby (there's a little pic in my sig link if you'd like to see it). Teeny tanks sometimes a pain to maintain, but so much fun to watch
Check out the gallery at www.nano-reef.com for inspiration, there are a few people over there with small hex tanks.
Already got a mantis tank. I was kind of thinking about a goby shrimp w/shrimp. I like blennies and gobies, but I don't want him to be the only thing in the tank (I know they don't get bored, but I would feel bad anyway) I like the hermit idea. Any more?
The cute one I have witnessed was a 5 gallon I believe. There was no fish but they had miniature pieces of coral and electric blue hermits. It was plumbed to a bigger system to keep salinity stable. They had pieces of xenia that was only about an inch, mushrooms and the smallest piece of frogspawn I have ever seen. If you can stabilize salinity the corals should be fine.