I probably wouldn't go with Celebrity unless the 'excellent food' you are looking for is tops on your list. Celebrity tends to draw an older crowd particularly so on longer voyages.
I've been on the three major lines and liked Princess the best. The Caribbean Princess is themed to sail in the Caribbean and as such has a more vibrant decor than Princess normally does. We found our orange and pink stateroom on Carnival a bit shocking.
Most people seem to recommend the Voyager class RCI ships as Raoul did. I was on a smaller RCI myself so I can't comment. Those voyager class ships seem to have a lot of amenities and activities on board for more active lifestyles.
Good luck, you're sure to enjoy it!