I have used the excercise room equipment-treadmill and weights. The aerobic classes are on a sign-up basis w/limited space, so if you are interested do that the first day. Take the spa tour on the first day to findout what is offered and sign up then. Also, walking the deck is great.
I found that if I went light on lunch it helped. I avoided the lunch buffet because I love breakfast and the whole social atmosphere of dinner. Lunch was substituted w/ some sort of snack and a pina colada or those ohter really good pink drinks-Bon Voyage I think they are called.
So drink a Pina Colada, while walking deck 4 and you should be even!!
HAVE A GREAT CRUISE!!!