Totally agree with the pom pom (boxer) crab. They are SUPER cute, with those little pom poms. We had one at the LFS I worked at, and, when I scrubbed down his modified critter cage, the little guy would shy back under a bit of liverock. But, if my hand or scrub pad got too close, he'd wave his pom poms frantically. So, freakin' adorable! Especially when they use their anemones to grab food! They are also very pretty with pink, white, and purple-black markings.
Pin cushion style urchins (like the black tuxedo urchin) or pencil urchins are pretty neat, and don't get as large at the long-spined urchins. However, any of the pin-cushion style of urchins (like the black tuxedo or the hairy pincushion urchin) will pick up rocks, debris, macroalgaes, etc. to camoflage themselves. This can be fun, or utterly annoying. My friend used to love sticking little clumps of caulerpa on hers to make them look like they had little green wigs. I had one at work grab a tuft of Xenia (and, strangely, the Xenia survived and grew!). If you have small frags, they may grab them and pick them up (*you'd be surprised at what a persistent little guy a pincushion can be when they want to carry something).
I like a nicely colored emerald (mithrax) crab. Once adjusted to a tank, they are not too terribly shy compared to some other crabs. However, they can be a bit dodgey from time to time if you plan to keep corals. You can sometimes find specimens that seem to derive great joy in plucking up all of your polyps.
As per shrimp are concerned, there are a few different options. Coral banded shrimp, fire cleaner shrimp, and skunk cleaner shrimp are all gorgeous animals, with bright, crisp coloring. The sexy shrimp has fascinating behavioral patterns (c'mon, who here can deny the fact that a shrimp that sticks its butt in the air and wiggles it ISN'T a cool conversation thing?). The pistol shrimp makes neat clacking noises, digs burrow-holes, and can come in some cool colors, but can be a bit reclusive. Shrimp can vary, so comtemplate what would be best for your situation (such as, if this tank is in your bedroom- avoid pistol shrimp)
G'luck! As we all know, 3G and smaller picos have a special place in my mind. Hopefully, you will have as much fun, if not more fun, that I have had with mine.