I love cruising and go once a year so I definitely understand the worry about overindulging.
I think your plan is similar to how I approached it last August. At dinner, my travel partners would order 3-4 entrees EACH and gorge whereas I ordered one thing and my bf ordered another and we shared it so we could taste both without over-doing it. Sharing plates let me taste everything without forcing myself to eat everything or feeling left out.
DEFINITELY take advantage of the gym! The ship I was on also had a steam room and it was so relaxing at the end of a workout just to sit there and feel like you are getting a free facial
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I'm going to resonate what other people have said
-Take stairs instead of elevators (It's faster anyway....elevators are ALWAYS slow)
-Do lots of walking to enjoy the view
After a few times cruising, I found that the food sometimes gets repetitive (especially the lunch buffet) so the first day you are overwhelmed with the choices but it's often repeated (at least a portion of it) during your entire stay. Don't feel overwhelmed to eat everything on the first day (I had that experience when I went on the first time
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ENJOY YOUR CRUISE