Royal Caribbean's Liberty of the Seas features the FlowRider, a surfing machine that poses a challenge that many youngsters can't resist.

How do you make a sister ship different from the original?

If the original’s a winner, says Richard Fain, chairman and CEO of Royal Caribbean Cruise Line, you don’t.

That’s why the new Liberty of the Seas is almost a clone of the line’s Freedom of the Seas, which debuted as the world’s biggest cruise ship last year.

‘‘We were happy with Freedom, didn’t see any need to make changes,’’ Fain said on a pre-inaugural cruise to introduce the 160,000-ton Liberty to travel agents and members of the media.

Dining venues, shops, lounges, the theater, sports and fitness areas, the spa and other facilities all duplicate those on Freedom, except for one minor shop replacement.

What IS different, Fain said, is the art aboard the vessel. This $7-million collection is extremely varied, ranging from Michelangelo collages and mythical figures to sports art and giant tile mosaics of toucans. In sum, though, it coalesces into one of the most attractive displays this observer has seen on any cruise ship.

Most striking is ‘‘The Ribbon of Life,’’ a group of six works by Miguel Chevalier and Arik Levy that combine sculpture and digital art. Suspended vertically from the ceiling of the Promenade, four large sculptured ribbons are illuminated with changing electroluminescent lights, evoking images of nature. The two other ribbon sculptures are horizontal, with motion sensors that allow guests to interact with them.

And just for interactive fun are two small works, ‘‘Squid Attack’’ and ‘‘Alien Abduction,’’ fitted with tiny cranks that allow guests to make the figures inside the artwork move.

As on the Freedom, however, art plays a background role to the variety of activities available to guests.

FlowRider, the onboard surfing simulator introduced on the Freedom, always draws a patient line of wannabe surf heroes. It’s a challenge most younger guests can’t resist, even though most of them fly ingloriously off their boards within a few seconds.

But then, they can soothe their pain with a cold one and a T-shirt at the nearby Wipe Out! bar. Or restore their bruised egos by climbing the ship’s enlarged rock-climbing wall, choosing from 11 routes to the top.

Some young children try the FlowRider, but most head straight for the H2O Zone, a marvelous water park providing all sorts of liquid fun - water to slosh in, to squirt at, to shower under and swim in.

‘‘H2O is the greatest. You can spend all day there,’’ said Mary Lou Ruderman, a passenger and chief financial officer of a online vacation company based in Plantation.

Then there’s the ice-skating rink and ice show, first seen on the line’s Voyager-class ships. The show still draws raves, and for good reason - the performers do a sparkling job in a relatively small space, and spectators go away wondering how the skaters can make such rapid costume changes while on the ice. When it’s not showtime, passengers are welcome to try their own hand - or feet, we should say - on the ice.

Also repeating from its Freedom debut: the Everlast boxing ring, a good place to work off hostilities.

For this pre-inaugural cruise, Royal Caribbean set up a one-time nighttime event on CocoCay, its private island in the Bahamas. Normally, passengers are ferried to the island and return to the vessel in late afternoon. This time, a special evening concert by Jon Secada was staged on the island. Back on board, waiters set up tables on the pool deck and the one above for a barbecue dinner and fireworks show after the concert.

Both events - firsts for the line - were so well received that Royal Caribbean officials said they may consider staging similar night events on CocoCay.

Also introduced on this voyage was an enhanced wellness program called Vitality, which replaces the Ship Shape Dollars program. The new program incorporates all aspects of wellness, including sports, fitness, spa treatments, dining and food choices. Participants can earn points and exchange them later for a logo gift.

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LIBERTY OF THE SEAS

-Length: 1,112 feet

-Width: 185 feet

-Cruising speed: 21.6 knots

-Passengers: 3,634 double occupancy

-Staterooms: 1,817 (1,084 ocean view, including 842 with balconies)

-Itinerary: Liberty will alternate seven-night eastern and western Caribbean cruises, departing year-round from Miami on Saturdays.

-Fares: From $649 inside, $849 ocean view, $948 balcony (lowest fares usually available only on low-season sailings).