Culebra, an 11-sq-mile slice of sleepy eccentricity, is revered by tranquillity seekers, while 52-sq-mile Vieques caters to a wider variety of tastes, from families to night owls.

Floating in a political no-man’s-land between US statehood and outright independence, Puerto Rico is an archipelago made up of four significant land masses, only three of which are inhabited: the heavily populated main island and its two eastern siblings, Vieques and Culebra.

Divorced from the clamour and commercialism of the crowded “mainland”, the latter two islands – both reachable by a 90-minute boat ride from the port Fajardo – purposefully shun Puerto Rico’s default attractions of golf and gambling. The island’s lay claim to broad unblemished beaches, vigorous expat communities and a spirit of seditious nonconformity, borne in part out of the islands’ recent histories as firing ranges for the US Navy.

By far the smaller of the two islands, Culebra is Puerto Rico at its most esoteric, an 11-sq-mile slice of sleepy eccentricity, where clucking hens wander down potholed side-streets, shops hang signs saying “open some days, closed others” and the nightlife begins and ends with a crowd of bearded Hemingway-look-a-likes at the waterside Dinghy Dock bar.

Wilder and more bucolic than Vieques, Culebra is favoured by self-sufficient types who can survive when the island’s limited ATMs run out of cash. It is also revered by tranquillity seekers. Life beyond the weathered “capital” Dewey rarely gets out of first gear, and the most popular form of after-dark entertainment is counting stars.

Insulated in their idyllic domain, Culebrenses are less effusive and more backwards-before-coming-forwards than other Puerto Ricans, but recalibrate your body clock to off-beat “island time” and you will quickly find them to be hospitable and bitingly funny hosts.

For lunch, head over to Barbara Rosa, an amiable señora who runs a restaurant out of her front room. Grab a menu, shout your order through the kitchen hatch, and wait on the front patio with a dozen or so of Barbara’s cats for the magic to materialize. For dinner, sample bold culinary creativity at beach shack cum no-frills guesthouse Mamacitas in Dewey.

Culebra’s finest attractions are first and foremost of the natural kind. Turtles come to nest here each year, a rare lizard found nowhere else in the world stalks the bushy slopes of Mt Resaca, and the island’s lack of significant river run-off has created an unusual level of water clarity, meaning water-orientated activities reign supreme. Divers love it, as do snorkellers and boaters intent on exploring some of Culebra’s 20 surrounding islets.

But the island’s trump card lies two miles north of Dewey, at platinum-blonde Flamenco Beach -- a rustic but justifiably popular scimitar of sand hailed in tour brochures as the best in the Caribbean. Campsites rather than beach condos lure romantic overnighters.

Covering 52-sq-miles, Vieques plays Scotland to Culebra’s Wales. But, while larger and more heavily populated than its errant northern twin, its idiosyncrasies are gentler and a little easier to digest. Wild horses rather than rare lizards pepper the grassy pastures, and a smattering of boutique accommodations have lent the two settlements of Isabel II and Esperanza an upmarket sheen. And the island’s refreshing dearth of traffic (or a single traffic light), makes it ideal for cycling.

With more space and a better infrastructure, Vieques caters to a wider variety of tastes than Culebra. You can “slum it” in a quintessential Caribbean beach shack, or clink cocktail glasses at the W Retreat and Spa. The island also attracts families with its profusion of beaches (40 to Culebra’s half dozen) and safe road-rage-free streets that are eerily reminiscent of the US sixty years ago. Though soporific by “mainland” standards, Vieques retains a pulse after dark. Regular live music lights up sultry evenings in Esperanza and Isabel II.

The pièce de résistance, however, is Vieques’ dramatic Bioluminescent Bay, an ethereal lagoon full of tiny micro-organisms which glow purplish-blue when disturbed after dark. The effect is rendered all the more psychedelic thanks to Vieques’ welcome lack of light pollution. Try kayaking the bay at night or taking a tour in a specially-designed electric boat.

Vieques’ military occupation lasted three decades longer than Culebra’s, with prickly public protests finally sending the US Navy packing in 2003. But beneath the ongoing clean-up campaign hides an unusual blessing. Dozens of beaches that were inaccessible to the public during the military episode have since been commandeered by the US Fish and Wildlife Service, meaning they remain virgin and unsullied by resort development. Angling off the rarely-fished southern coast is phenomenal.

The article 'Culebra vs Vieques: Puerto Rico’s contrasting islands' was published in partnership with Lonely Planet.