When a landmark birthday or anniversary rolls around, why throw a simple dinner party when you can rent a private island, fly your friends to Paris, or throw a party in Las Vegas for 200 guests -- all dressed as Elvis or Priscilla Presley. The latter is what Paul Kanarek, owner of the Princeton Review of Southern California, a test preparation firm, did for his 40th birthday. He rented a mansion on the outskirts of Vegas, hired a band to play all Elvis tunes, and rented Presley costumes for the entire group. Later in the evening, the Kings converged on the Paris casino to gamble. "I recently went back to the Paris," says Kanarek. "The staff is still talking about the night of the Elvises."

Extravagant celebration has often followed business milestones -- the first big contract, an infusion of venture funding, an initial public offering. But sharing personal milestones with family and friends in exotic ways has been on the rise lately. In a survey by travel services marketing firm YPB&R, 54% of respondents cited "spending time with friends" as extremely important to their travel, up from 43% in 2000. The firm calls the trend "togethering." A new travel website, Groople.com, is entirely devoted to the phenomenon and designed for easy booking of groups traveling together.

You don't have to be married to a Tyco CEO to want to have a memorable private party (though we'd advise against using company funds or wearing gold lame hot pants). Pat Gallagher, founder of financial analysis firm Black Diamond Research in New York City, is planning a joint celebration of his 40th birthday, his wife's 30th birthday, and their 10th wedding anniversary by renting villas for 20 guests at Casa de Campo, an upscale resort in the Dominican Republic. The villas have private pools and maid service, and meals and booze are included. While at the resort, "none of my guests has to shell out any cash," says Gallagher.

Barge cruises in Europe are a popular choice for small groups. These typically fit four to six couples, and rentals include the whole boat, captain, chef, and mates. Matt Rosenthal, owner of Stratford Management in Boston, went on a barge cruise through the Loire Valley in France with two other couples for his friends' 10th anniversary. "We floated down the canal, drank wine, ate, and visited castles," he recalls. "It was a great bonding trip, and we never had to think about logistics."

For those with even swankier tastes, there is always the ultimate party site, the private island. A growing number of them are being rented out for 30th and 40th birthdays, says Janine Cifelli, vice president of Sanctuare, a niche representation firm for exclusive properties worldwide. Its flagship rental is Richard Branson's Necker Island in the Caribbean, which accommodates 26 and costs up to $40,000 a night. Sanctuare also represents slightly less pricey ones such as Musha Cay ($24,750) and Guana Island ($15,000).

Drew Rayman, owner of i33 Communications, a New York City-based Internet marketing firm, has rented Necker on four different occasions, once flying in 26 friends for a party. "I come from a middle-class background and a lot of my friends could never, ever have experienced anything like this," says Rayman. "It blew them away." And that may be the greatest compliment a host can receive.

Planning a special occasion? Here are some places to start

Abercrombie & Kent: Guided private adventures such as treks to the Himalayas, African safaris, and everything in between. 800-554-7016, www.abercrombieandkent.com

The Barge Lady: Private barge rentals in Europe. 800-880-0071, www.bargelady.com

Cruise Professionals: Cruises for up to 600 people. 800-265-3838, www.cruiseprofessionals.com

The Moorings: Crewed sail and powerboat charters worldwide. 888-952-8420, www.moorings.com

Mustique: A haven for rock stars in the Grenadines, this island has 85 private homes, most of which are for rent and have as many as nine bedrooms. 800-747-9214, www.mustique-island.com

Sanctuare: Private-island rentals, like Branson's Necker Island. 800-225-4255, www.sanctuare.com

Total Entertainment: An event firm that has thrown some of the biggest bashes in the world, including Paul McCartney's wedding. 201-894-0055, www.totalentertainment.com