Tim, head over to Pelham bay and pick up some starfish, you may even find a horseshoe crab. The starfish you find at Pelham are real colorful and cool. Cold water, but if only for a day it should be fine.
The touch tank at the Scripp's Aquarium in San Diego had long spine urchins. It was really cool. They let the kids put their finger between the spines and told them it was an urchin handshake when the spines moved around their finger
They also had some huge sea cucumbers that they let the kids (and me) pet, but I have no idea what kind they were.
That's not a bad idea. Cucumbers, Starfish (check with my dad- he has the red chocolate chip star still I think), urchin and maybe a clam or something... (Not a tridacnid)