Here is a couple of tips and tricks to have a healthy vacation (not that I exactly followed all of this on my trip - shhhh, don't tell my trainer).
1. Buffett = Bad. Most people tend to overeat at these bad boys. Stick to cereal/fruit for breakfast. Lunch? Salads are awesome - avoid the creamy dressings. And, do not put guacamole on everything. (Again ... shhh).
2. Pool Bar. Avoid the blended mix drinks. Pina Colodas are the worst of the horrible blending concoctions. You should avoid anything that is 3 colors as well.
3. Tennis shoes. Bring them and use them. Do not be a lazy A$$ because the gym equipment is not functional. Walking on the beach is a great way to excercise as sand provides resistance.
P.S. Snorkeling is not real exercise either.