A different type of cruise experience: finding balance and vitality on Royal Caribbean's Liberty of the Seas.

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I'M HALF ASLEEP ON THE BALCONY OF MY OCEAN VIEW stateroom when a lilting, accented voice over the intercom urges passengers to action--either for a 6 p.m. early dinner or for a fitness class. Our itinerary on Royal Caribbean's Liberty of the Seas includes stops in Puerto Rico, St. Maarten, and Labadee--Royal Caribbean's outpost on Haiti--with a lot of onboard action in between. But right now the question is what to wear?

The Liberty is one of Royal Caribbean's largest ships with 1,817 cabins, a golf simulator, and an outdoor sports court. There are so many dining and entertainment options spread out over 15 decks--and wellness activities to undo the damage--that I'm truly stumped. Do I show off an impressive resort ensemble in the solarium hot tub or strap on my new cross-trainers? Should Jen (my sister and cabinmate) and I check out Sorrento's for 2 a.m. pizza (chances are we'll never meet the other 3,632 passengers again anyway), or do we turn in early for an 8 a.m. jog around the Sky Bar deck (followed by a Ben & Jerry's blueberry waffle cone, naturally)?

Ah, but the scales have been tipped toward fitness with the ship's new vitality program. Created in partnership with New Balance, its goal is to help passengers achieve "total wellness in mind, body, and spirit" during the seven-day cruise and beyond. The campaign is supported by seminars such as "Eat More to Weigh Less," great international cuisine (grilled lobster tail, Jamaican food cooked by Jamaicans), and even a sporting-goods shop--right across from the chocolate cookies at Cafe Promenade. Yes, it's all about balance and competition. With sibling rivalry big in our relationship, Jen and I--and a number of other cruisemates--compete for Vitality Rewards. Attend any number of events and seminars with the Vitality logo, and you'll earn points that can be traded for prizes such as backpacks and T-shirts.

DAY 2

A hangover nixes the 7 a.m. Sunrise Stretch, and we aren't mentally ready for 10 a.m. tai chi at the ShipShape Fitness Center, so it's a light breakfast at the Windjammer (miso soup or waffles, anyone?). At "Health & Physical Fitness," we are reminded that vegetables are good and too much fat is bad. To show her dedication, Jen gets pricked during acupuncture, while I get pedicured at the Liberty Day Spa. The jabs continue at dinner in the main dining room, where Jen goes for the poached salmon Vitality option. I choose duck a l'orange.

DAY 3

The 8 a.m. New Balance Walk-A-Mile powerwalk has us lapping a variety of fit types on the Deck 12 track as we dock in Puerto Rico. We window shop in Old San Juan and fill up on tamarido snow cones at the famous El Morro fort. But the intense afternoon heat chases us to rum and Cokes at a bar that also sells bikinis. Back onboard, it's the casino and casual dinner, where a 14-inch plate can do a lot of damage at the Asian buffet.

DAY 4

At breakfast, we tell ourselves we're "carbo-loading" with waffles, sausage, bacon, fruit, yogurt, and smoked salmon for cycling in Dutch St. Maarten. However, the slow pace of the Bike Tour Shore Excursion has us drinking in the local flora, fauna, and folks along the way. We end up cooling off with rum punches and a dip in the ocean at a beachside cafe run by friendly French folk.

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DAY 5

We're invited to a wedding at the FlowRider surf simulator, followed by the reception at the private Viking Crown Lounge, culminating with tonight's Venetian feast. So we eat, and eat.

DAY 6

In Labadee, the Dragon's Breath Flight Line is reportedly the longest zip line over water in the world. We should get Vitality rewards for just climbing the hill to get to it and for the rush of adrenaline as we speed down over the Caribbean Sea to the jolting halt at the other end. We're soothed with yoga on the beach and barbecue.

DAY 7

It's the last day and we're making it worth our while. We walk a mile, attempt tai chi, and the indoor cycling class gets us ready for a soothing aroma spa seaweed massage. Jen and I load two backpacks and water bottles, and she edges me out with a Vitality logo bottle. But I don't care--it's all about the balance, isn't it?