Remember how much fun it was to climb trees?

Well, some adults are doing more than just waxing nostalgic over their sun-soaked afternoons in nature's playhouses. They're actually paying to relive them.

Tree-climbing courses — or guided tours for grownups who can afford to monkey around — are gaining popularity around the world.

"People are climbing all over the place," said Debbie DeLisle, a vacation planner with Appalachian Vacations & Adventures in Hot Springs, N.C.

The courses, however, don't entirely re-enact the days of swinging from branch to branch with light-footed abandon. Students are saddled, harnessed and tied with rope for the duration of their climbs and descent.

But that doesn't mean you don't have to watch your feet.

"It's a matter of 'where can I step next?' said Fernando Perez, a tree-climbing guide with the Costa Rica-based Serendipity Adventures.

Aside from the physical feat of the climb, which can span heights of up to 100 feet, there are other obstacles to endure. Many tree schools have their students climb up within a hollowed-out "strangler" tree — where the humans are merely the visitors.

"There are bats in there, monkeys, butterflies," Perez said.

The tree-climbing boom is largely practical: Climbing gear has become much more user-friendly in the last couple of years, DeLisle said. But the roots of this Swiss Family Robinson redux run deeper than mere accessibility.

"The fact that people are spending more time in the office has led to a rising interest in outdoor recreation — people are looking for a chance to get out of the city," DeLisle said.

In fact, most climbers are white-collar professionals tired of sitting on their you-know-whats. And it seems the events of Sept. 11 have inspired new students to seek Edenic refuge in the great outdoors.

"People are saying 'I just need to get away from the TV and radio — to get completely away from it,' DeLisle said. We have seen a lot of folks from New York lately."

And why pay for a room with a view when you can experience wilderness first-hand?

"I saw birds, wildlife, big tall trees — it was such a rush," said Pat McArthy, 54, a Georgia-based bank vice president who climbed with Serendipity in 1999. "And I felt really proud."