With all those stairs, it's really hard to gain weight. It's up to the Windjammer for breakfast, then down to the trivia contest, then up to the pool, then down to the room to freshen up, then up for lunch, then down for the guess that tune show, then up to the pool, then a walk around the deck, then back down to freshen up before dinner...etc. etc.
As for making the right choices about what you eat, well, that's just something you have to decide to do yourself. Try the more sensible main dining room for breakfast and lunch. Just have some fruit and coffee for breakfast, and the salad bar for lunch. Keep using the stairs all day, then enjoy a nice dinner.