There's a hot tub in the Castaway Cove play area-- ahhh, we loved it when we were there!
About checking in: you can check in as early as you want, but you may not be able to get into your room until 3 p.m. If you get there before then, you can check in, and they will store your bags for you in their storage room. Then when you go back later, they'll give you your room key.
The reverse is true, too... if you are checking out, then spending part of the day in the parks before taking off, they'll check you out and then hold your luggage for you. Works great! They were so nice to us, on our last day we checked out at 7:15 a.m., and the guy said, "Just leave your bags, and go ahead so you can get to Magic Morning. I'll put them in the luggage room for you." Then we went back at 3:00 to catch our shuttle to the airport.