The 4-foot-tall yellow sign on the chain-link gate didn't just say, "Danger: No trespassing," in block letters. It also had a black-and-red image of a bomb and the warning "Explosives -- unexploded ordnance" in English and Spanish.

Nevertheless, I squeezed though the gate's opening. My husband, Andrew, followed me down the path to Carlos Rosario Beach.

We didn't necessarily need to take such measures to find solitude on Culebra, part of a 23-island archipelago off the eastern coast of Puerto Rico. About a quarter of the archipelago is a U.S. Fish and Wildlife Service-administered national wildlife refuge, much of it off-limits. Until the mid-1970s, the U.S. military used some of the islands for gunnery and bombing practice. (Culebra's larger neighbor to the southwest, Vieques, site of the former U.S. Navy base, is more developed and better known.)

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Beautiful and relatively undeveloped, the island provided a welcome respite for a long weekend. And because we were willing to take the trails less traveled (we checked with a park ranger before ducking past that warning sign), we were able to go from one serene setting to another. Though we did Culebra off the grid, you needn't be an adventure traveler to enjoy the island's charms. Don't expect luxury resorts as on Puerto Rico, but a family can enjoy a comfortable, no-frills vacation here.

Vacationing off the grid

We'd rented a one-bedroom hilltop home in southwestern Culebra, attracted by the property listing noting that the villa was entirely off-grid. Indeed it was: Three tanks collected rainwater, and solar panels provided electricity. Because we had only sporadic mobile access, I abandoned my phone in a drawer.

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We accessed our little villa by a driveway so steep that our Jeep seemed ready to tumble into sailboat-flecked Fulladoza Bay below. It was far enough away from Dewey, Culebra's one small town, that peeping coqui frogs provided the only nighttime sounds.

The next morning, we headed down the precarious driveway and through Dewey to the northwestern corner of the island, where a nearly empty parking lot and a ramshackle concession greeted visitors to Culebra's most famous landmark: Flamenco Beach.

Flamenco is undeniably gorgeous. It regularly appears on top-global-beaches lists; in February, TripAdvisor rated it the world's eighth best. Its mile-long white crescent hugs a clear, calm bay. Only the concession and a small hotel interrupt the palm trees flanking the sand. Pelicans and boobies dive into the water.

Even when busloads of weekend day-trippers unload onto Flamenco, there's room for everyone. During our weekday visit, perhaps a hundred people spread across the sand.

At Flamenco's western end, two old tanks, relics of the military exercises, rest on the beach. Graffiti has turned these once-lethal hulks into surreal art installations, a colorful reminder of the island's controversial past.

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Guidebooks mention the trail to Carlos Rosario Beach, but most visitors don't notice the gate tucked into a shady corner of Flamenco's parking lot. In my clumsy Spanish, I asked a nearby ranger whether I should be worried about the "Danger" sign.

Although some rangers recommend a longer shoreline route that is definitely free of explosives, he responded (in smiling English): "Go right ahead -- you will be the first ones at the beach today. It will be hot, so make sure you have plenty of water." Beyond the gate, the path to Carlos Rosario was an easy 20 minutes. We were careful not to stray into the bushes.

We were the only people on the paradisiacal shoreline. We walked along the water's edge, along piles of sun-bleached coral and black rocks, until we found the perfect tree. We laid a couple towels in its shade. Later, a dozen other visitors would join us, but for now, we had our own little piece of the Caribbean.

Getaway by kayak

The next morning, we headed across the island, driving past a wildlife refuge and houses scattered among the hills. At the Zoni Beach entrance, hand-painted signs warned nighttime visitors away from the nesting areas of leatherback and hawksbill sea turtles.

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A local outfitter had dropped off a two-person kayak for a self-powered day trip to Culebrita, a one-square-mile island, less than two miles off Culebra, that's part of the national wildlife refuge. Culebrita Lighthouse, one of the Caribbean's oldest, crowns its top in decrepit glory.

As we paddled out from Zoni, a salt breeze rippled the water. A magenta jellyfish pulsed at its surface. Fish darted under the boat. We glided over reefs, watching corals and sea fans pass beneath.

Only a handful of people visit Culebrita each day, and today was no exception. We pulled our boat onto an unclaimed beach fringed with dense trees that climbed the slope to the lighthouse a few hundred feet above, hidden by the foliage. It was an ideal hideaway for a midmorning siesta.

On the return to Zoni, the wind strengthened, pushing us toward Culebra. Lingering in the swells, we waited for a break and paddled for the beach. Then a wave rose under me. I hovered on its crest for one dreadful moment, and the bow surged into the air and plunged into the water. I flew off the boat and the wave crashed over my head.

A nearby family, picnicking on the now-populated beach, watched with amusement as I slogged onto the beach and sat down in a sodden but contented heap.

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It was an ungraceful tumble back into civilization -- and probably good practice for returning to the rough waters of non-Culebra life.

IF YOU GO

GETTING THERE Cape Air offers four daily flightson smallplanes from San Juan, about 35 minutes one way, starting at $200 round-trip. A ferry from Fajardo, on the east coast of Puerto Rico, is $4.50 round-trip, with a taxi from San Juan to Fajardo costing about $80. capeair.com, islaculebra.com/puerto-rico/culebra-ferry-schedule.html.

WHERE TO STAY Club Seaborne is a clean, well-reviewed hotel with rooms and villas overlooking the bay, free daily breakfast plus kayaks and bicycles for guest use. Doubles from $169, 787-742-3169, clubseabourne.com.

Check out VRBO (vrbo.com) and HomeAway (homeaway.com) for a list of vacation rentals on Culebra.

WHERE TO EAT Mamacita's Restaurant and Bar, 66 Calle Castelar. Get a seat by the stream at twilight and watch tarpons swim in the water and bats hunt for mosquitoes. Entrees $15-$25. 787-742-0090, mamacitasguesthouse.com.

Dinghy Dock, Calle Fulladoza. A popular bar and restaurant on the dock where the water taxi comes and goes. Steaks, hamburgers and daily seafood specials, $12-$30. 787-742-0581

Zaco's Tacos, 21 Calle Pedro Marquez. Tacos, burritos and more for less than $10. 787-742-0243, zacostacos.com.

WHAT TO DO Culebra Snorkeling and Dive Center, Plaza del Mercado. Offers kayak rentals ($40/day) and paddle board rentals ($75/day), as well as tours and a retail shop that carries beach and snorkeling gear. 787-435-3662, culebrasnorkelingcenter.com.

Culebra Divers, Calle Pedro Marquez #4. The dive shop across from the ferry terminal offers snorkeling trips (from $60), scuba trips (from $85) and scuba classes. 787-742-0803, culebradivers.com.

INFO islaculebra.com