Actually visit the fitness center. You don't have to be "hard body" to use a treadmill or elliptical. Fifteen or twenty minutes at 2.5 or 3 mph will do a world of good and the view is usually one of the best on the ship. Just make sure you don't have any medical issues that stop you from doing this.
Use the stairs when you can, even if it's just going "down".
Sit as far away from the buffet as you can. You'll be less likely to go back for seconds.
Scope out the buffet before you load up your plate, don't just dive in. Be selective.
If you REALLY want a second entrée see if someone will split it with you.
Also, remember: There is nothing wrong with not cleaning your plate. Your mom is not watching.