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I still can't believe that in 1975 my parents allowed me, an unworldly 18-year-old, and a girlfriend to take a weeklong camping trip to Virgin Islands National Park to mark my senior year of high school.. Two unaccompanied 18-year-olds, long before the advent of cellphones. But thankfully, they did. I've been back twice for reasonably priced "senior trips," when my two sons graduated in 2009 and 2013. It's now a family tradition. I'd wager we'd all go back tomorrow.

In 1956, John D. Rockefeller donated nearly half of the 12,500-acre island of St. John to the National Park Service, so the northern rim of the island is pretty much one public beach after another. And not just any beaches, but drop-dead-gorgeous, palm-lined, white sand beaches that regularly make island-calendar covers. And free, except for a $5 entrance fee at Trunk Bay, where the cruise ship passengers from St. Thomas descend. You simply rent some snorkel gear, hop the north shore open-air shuttle buses, get off at the beach of your choice, and wade into those generally tranquil and sparkling turquoise Caribbean waters.

Of course, it's not quite so simple getting there. St. John doesn't have an airport, so flights land in St. Thomas. From there, you take a taxi through the back streets for over an hour to the Red Hook Ferry. At Cruz Bay on St. John, you rent a car (drive on the left!) or hop on a shuttle. Note: Cruz Bay is where nearly all the hotels are, except for rental villas.

And then there's the campground at Cinnamon Bay, within the national park borders (cinnamonbay.com). It's a couple of bays down the north shore road, at the turnoff with the covered café dining spot.

"You camped in the Virgin Islands?" friends asked. It's not "glamping," but there is no need to haul gear. Canvas tents with floors, cots and picnic tables — our home-away-from-home in 1975 — are still similar today, albeit more in demand. Sandy trails lead campers to the shared bathhouse, the store, the café and some rudimentary cement-block motel units. A propane stove, utensils and cooler are also furnished at the campsite. Ice, food and other essentials are sold at the store, or you can shop in Cruz Bay, where prices are slightly lower but still island-high.

Cinnamon Bay is one of the broader beaches, and the waves tend to be a little wilder. That's where my first saltwater swim came with a painful tumbling into hard coral sand, because I underestimated ocean waves and undertows compared with the Great Lakes.

The whole park, which includes underwater reserves, inland mountains and restored ruins, is magical. Wild donkeys might pass you on the trails or beach or step in front of your SUV on a switchback. (The animals roam the island, descendants of donkeys that slaves tethered to millstones to grind cane into sugar for making rum.) Iguanas sun themselves on rocks and hermit crabs scurry through the leaves. The people are St. John Nice.

Take the road farther down to the last three north shore bays — Francis and Little and Big Maho — and the snorkeling gets better and the crowds thinner. Waterlemon Cay beach is rocky, rather than deeply sandy, but it's probably the best snorkeling on the north side (and there's a fun hiking trail there where you can see cactus and maybe wild goats). Salt Pond Bay on the south side, near the village of Coral Bay, also offers excellent snorkeling. Sea turtles, rays and barracuda sightings are common.

"What beach do you want to go to today?" was the most difficult decision of the day.

We also hiked some of the National Park trails. The popular guided Reef Bay hike leaves from the visitor center in Cruz Bay (sign up online weeks before you arrive, as it sells out. The hike is offered by Friends of Virgin Islands National Park; friendsvinp.org).

We were shuttled a couple of miles southwest to the trailhead, then hiked downhill through towering tropical forest with a ranger, who pointed out native flora and fauna. She also taught history by noting the ruined remains of fences and sugar mill plantations (other restored ruins are at the Annaberg plantation on the north side of the island) from when the Danes ruled, slaves sweltered and rum was the only export. The hike fee includes being met by a boat when the hike ends and being ferried back to Cruz Bay.

Other than swimming and snorkeling, you can rent kayaks, take a day or sunset sail, or join a Park Service volunteer work day, repairing a trail or rebuilding a bridge. St. John and Coral Bay have laid back open-air restaurants and bars.

Sadly, Maho Bay Camp, where thousands vacationed in hillside camper cabins connected by boardwalks, was sold to a private party and closed in 2013. That makes demand for the Cinnamon Bay campground higher, so book early. But fair warning: Once is not enough for the St. John park magic!

Laura Zahn is a financial adviser in Duluth.