Have a great time! I remember our first cruise just a few months ago and how totally excited I was about going, and how I obsessively read these boards (and still do! gotta relive the magic, ya know?).
Take some time Saturday afternoon to explore the ship and get your bearings. Don't miss the SailAway Party (go up a little early for a spot right in the fray or to get a good spot on Deck 10).
Enjoy Sunday morning on board before porting in Key West. Go out on the top decks to watch the approach to Key West. Eat lunch before going out for the afternoon.
Be sure to check out Diversions during the day - it's a nice, quiet place to relax away from the crowds. We didn't "discover" it until half-way through our cruise and wished we had earlier.
Make sure to talk to the Cruise Staff - they are a lot of fun.
If your group on the Cruise Meets board is planning a meet, be sure to go == it's nice to have familiar faces around the ship.
Take a walk on Deck 10 (just be prepared for how strong the wind can feel).
Play ping-pong. Let the little kids chase the balls. Try your hand at shuffle board even if you don't know the rules
Watch the sunset as many evenings as you can ... and watch at least one sunrise (you can always go back to bed).
Take a nap in the morning and in the afternoon, and maybe one more before bedtime.
Don't think about how many days you have left (it gets depressing when you pass the half-way point). Enjoy each day to its fullest.
Have a MAGICAL time!