Right now the Rhapsody is sailing to Key West, Cozumel, and Grand Cayman. In September or October, Grand Cayman is dropped for Belize. Also starting this fall, the Splendour will be sailing from Galveston on 10 day itineraries. I believe the ship will alternate between a longer Caribbean route and a partial transit of the canal, but I'm not positive about that.
Carnival is also sailing out of Galveston with a couple smaller ships...the Celebration alternates five day and four day sailings. The Jubilee is going to be added soon, with a larger, newer ship planned for the future.