In other groups, each family books its own reservations.
Occasionally grandparents foot the entire bill. Martha and Ed Monaco of San Antonio just returned from an Alaskan cruise with 17 relatives, paid for by Mr. Monaco's parents to celebrate their 60th anniversary. His parents also hosted trips to mark their 40th, 50th, and 55th anniversaries. They choose the cruise line and destination. Each family pays for its own shore excursions and gratuities.
"My darling mother-in-law said, 'We decided we'd like to enjoy some of your inheritance with you,' " Mrs. Monaco says. "Cruises work out nicely because you don't have to worry about food. Everyone can do different activities. Teenagers can sleep late, and parents can stay up late."
For some families, adventure is the lure. "Over the last two to three years we have seen a noticeable increase in large family groups," says Steve Markle, marketing director for O.A.R.S. in Angels Camp, Calif., which offers river rafting.
His family groups average 16 to 20 people. "About 40 percent have never done anything like this," he says.