Another urchin vote here - they may move slowly, but they're still lots of fun. I love checking the tank every morning to see what hats our pincushion Speedy is wearing that day - sometimes it's a single shell, sometimes he's covered in bits of rubble, but the best days are when he picks up nassarius, cerith or even Astraea snails, or hermits. Watching a hermit try to figure out what's happening is just too funny

Speedy usually puts them down after a short time, probably doesn't like the feeling of his 'hat' struggling to get away.
My mother in law came over the other day, and couldn't believe her eyes. 'That thing is ALIVE? What do you mean it has little feet all over its body? How is it holding onto that shell? Can it see me?'
She ignored corals, the brightly coloured fish, a huge CBS, and just watched the urchin for about 15 minutes. How's that for entertaining?
Canarygirl - IME urchins aren't at all harmful, but you do need a glass tank (they can scratch acrylic) and frags that aren't epoxied down may end up being worn as hats.