Gregory has two castles. One to swim around, and one that he can hide in. I found a lighthouse thing at work the other day, and it has a hidey hole, so I decided to stick that in there, too. He hasn't gone near it yet. It usually takes him a few days to get up enough courage to go inside them. I think he's afraid of the dark. xD
I have some java moss in there, but he isn't interested in even touching it.