I’m thinking of going (alone) to the Caribbean for about 10 days in late summer, and I’ve narrowed it down to Puerto Rico and the U.S. Virgin Islands. I’m primarily interested in nature hikes (and drives) and cultural history, and most of all, photography. What I’m ***not ***interested in are: lazy days at the beach, water sports, casinos, rowdy nightlife, and sitting around doing nothing.
So is there enough in either place to interest me for 10 days, or should I split the trip between the two?
I know the island of St. Croix in the U.S. Virgin Islands well. My in laws have a house there and we go there most years and sometimes multiple times.
As you may know, there are three USVIs: St. Thomas, St. Croix, and St. John. They are all beautiful but very different. St. Thomas is the touristy one and has the big nightlife. You sound like you aren’t into that.
St. Croix is the largest one (I think it is about 30 miles long). It is largely pretty rural but it has two sizable towns (Christiansted and Fredericksted and opposite ends of the island). Frederickstead is not so nice and I would stay near Christianstead if you go. It is a quiet island but there are some nice restaurants, bars and things spread out with interspresed goat and cow fields. It has a Dutch colonial history and has some beautiful and distinct architecture including old stone sugar mills. You can just walk around in old buildings, fields and everything else. It is very laid back and people are used to it. The beaches are beautiful and numerous with crystal blue water and are usually almost deserted. There is a rain forest in the center of the island with plenty of space. You can easily hike around all over the place. The culture is largely slave descended black Caribbean with a small white ex-pat (not the right word) community. The only underwater National Monument is about a mile off of ST. Croix and it is called Buck Island. If you are into underwater photography but not a real scuba diver, it is the place for you. Small boat tours can take you to world class snorkeling that easily equals other world-class tropical scenery at very low depths (5 - 30 feet).
St. John is not very populated at all and most of it is a reserved park. It is the smallest of the islands and I believe the most expensive to stay in. I have never been there but I hear it is a nature lover’s dream but it is a little too quiet for some. You could hike all you want and explore deserted beaches. I don’t think it has much of a native population but I could be wrong.
You can take seaplanes from the different Virgin Islands if you want for day trips. I believe you can even go to some of the non-U.S. islands for different cultural experience. There are a massive number of Caribbean islands in that area and many are 40 - 60 miles apart.
I have been to Puerto Rico briefly. It could be a state so it has everything. The culture is very different that the USVI and the demographics aren’t similar at all so that is something to take into account when you choose.
I spent several months in St. Thomas, but that was back in 1976 and I was more interested in chasing skirts and drinking rum than sightseeing. But the island is photogenic. You can probably still rent a motorcycle and putt around the place. Lots of butterflies as I recall and mongooses. There’s Frenchtown and some old buildings in Charlotte Amalie. Like any place, there are areas to avoid, and apparently St. Thomas has, sadly, become more violent and unpredictable, particularly at night.
Most of my family is from St Thomas. It’s a beautiful place though not quite as peaceful as St Croix or St John (or maybe it wasn’t so peaceful because I was around tons of extended family).
On St Thomas, one attraction is Coral World - an underwater observatory. It’s unlike any aquarium I’ve seen. Instead of bringing sea-life inland, they built the observatory about 100 feet offshore and underwater. Here’s the website.
Of course there are beautiful beaches and the harbor - which is stunning at night… but if you’re not into all of that - ya might want to visit one of the other two.
Vieques - a small island off of Puerto Rico has the intriquing bioluminescent bay. If you’re interested in natural wonders, you might look into that as well. I’m not sure what the tourist accomodations are like though… the rest of my family is from Vieques so we didn’t have to worry about where to stay. 
You could always visit more than one island if it’s in the budget. 
I’ve never been to the Virgin Islands, but I bummed around PR for a couple of weeks with a friend who was taking a few semesters at San Juan University. I spent most of my time handing out with her boyfriend, who was a local.
PR has a ton of the typical touristy stuff you’re not interested in, but it’s also got all of the non-typical touristy stuff you are interested in. The drive up to Arecibo should give you plenty of opportunities for nature photography.
Ditto on what Mrs Johnson said about Vieques. You might also want to check out Culebra, which is the next island out.
I’m of course not completely unbiased (see my location), but with 10 days’ time, I think you could do both, on something like 7 days PR, 3 days VI – you’d shop around for whether you’d be better off booking a 3-leg Home-SJU-STT-Home (or reverse order) air ticket, or a roundtrip to SJU or STT and then a local island-hop RT next door in the middle of the trip. and you should leave yourself some time to just rest.
USVI:
The US Virgins have been discussed above – I should add, that Charlotte Amalie (St. Thomas) does have some very interesting cultural history, what with the successive occupiers (it was even DANISH, fer goodness sakes!) and the trading freeport tradition (OK, yes, there will be the temptation to shop). I really dug the Red Fort and the oldest active synagogue under the US flag. St. John, of course, is the one that advertises itself as “eco” touristy and all about “nature” and yes it’s the least developed. Basically you’d use either St. T or St. C as your main inbound-outbound gateway, use local flights between St. T & St. J, and ferry between St. T and St. J.
One last warning: though it’s the “US” VI, they have kept on driving on the left side of the road. So do look twice 
As for my home isle…:
Outside Old SJ, a car is indispensable. And dust off that St. Cristopher medal, this is a race that learned to drive from Manhattan cabbies and to maintain the roads from the Boston public works department 
but at least we drive on the right…
Old-city San Juan, and Ponce on the opposite coast of the island, have cultural history out the proverbial wazoo. Old SJ is has fortifications including two huge forts and a city-guirding wall that are a US National Historic Site and a UN World Heritage Site, great harbor and ocean views from the city walls, ancient churches, small museums of various themes (Book, Pablo Casals, African Heritage, etc), art galleries, government buildings (The Governor’s Palace and Capitol are historic buildings open for tours in daytime – I’m quite fond of the Capitol, where I’ve worked 8 of the last 12 years), public sculptures, the main offices and shops of the Institute of PRican Culture and the Center for Caribbean Studies, all housed in fine architectural examples from the 1520s to the Deco age, it is an absolute must-see (and the best part, the Capitol/Governor’s Palace/Morro Castle triangle is all within walking distance for a healthy person). (BTW cheap way to get pictures of Old SJ from the waterside: take a ride on the ferry to Cataño and back – it’s cheap and you get to cross the harbor)
Ponce is a more 19th-early 20th century old-school city with a smaller and quieter (but also beautiful) historic district around the Plaza, with its own cultural museums and a world-class major Art Museum. Also around Ponce good sites to see are the Conservation Trust’s Hacienda Buena Vista, a restored old-time coffee plantation (reservations req. for English tour last I checked), and the Tibes native archaeological site (nothing on the scale of the sites on the continent, but still worthwhile for history).
For nature exploration, the Biggie in PR is the highland rainforest at El Yunque (“Caribbean National Forest”, to the US Forest Svce., Visitors Center 50 min E of SJ), with trails of various degrees of challenge, through various forest biotope levels.
At the opposite corner of the island, (1hr. W of Ponce) there’s the Guánica Biosphere Reserve arid tropical forest, an extreme contrast of environments.
30 min. E of El Yunque there’s the Reserve at Las Cabezas de San Juan, in Fajardo, also run by the Conservation Trust (reservatiosn, again), which includes paths and lessons around the mangrove ecosystem, a historic lighthouse that was a site for a Spanish-American War diversionary skirmish, and another bioluminescent lagoon. Fajardo is also the gateway point to the aforementioned side-islands of Vieques and Culebra and other minor offshore cays.
If you take the coastal road (PR 187) instead of the highway to/from El Yunque or Fajardo, you’ll drive through the Piñones-Vacía Talega coast, which will give you a good sense of what the area around SJ looked like before they plunked down all those condos and hotels righ at the shore, with impressive beach/ocean views (unless the surf’s so high the road is closed!); plus it’ll give you a chance to eat some roadside food that’s good for the spirit, if not for the cholesterol count, and you’ll be driving through the more culturally “African” part of PR.
Other scenic/photogenic locations that may be of interest include the Reserve at Cabo Rojo (Red Cape, Cap Rouge) with its salt marshes, lighthouse, and seaside cliffs, at the extreme SW end of PR; and the Camuy River Caverns Park and the Arecibo Radio Observatory in the inland North.
Do a search for the names of the islands mentioned and my own or Karl Grenze’s dopernames and you should find earlier threads on the same subject.
And oh, yes: I love our islands and peoples but I’m not gonna lie – it’s not Shangri-La. I applaud travelers like you who do not intend to have themselves isolated from their environs,and want to see what the place really has to offer. Whichever destination is chosen, the rule is the same as for any destination these days: a decent application of common sense goes a long, long way to prevent bad situations when moving through unfamiliar territory.
I mean between St. T and St. C or vice-versa. And to think I previewed twice.