I put a small, specialty type ornament in each stocking every year. This year I've already bought a glass crab for my son. I found ballerinas when my DD was still dancing, and mermaids when she was collecting them. DS has gotten turtles, frogs, and trains in the past. DH gets sports or dragon ornaments, or anything else that I think might amuse him.
They go on the tree, but will "go" with the children when they have their first home.