My suggestions for some sights in the DR, if you have time-
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Take the road to Sabana de la Mar (above Hato Mayor) early in the morning (like 7 or 8 am), and enjoy the peace and quiet - very much like mountain roads east of Seattle, Washington with low fog, etc. There is a stretch of this road that is lined for maybe 10 km with 50' oil palms on both sides of the road, as far back as you can see.
At Sabana de la Mar, you can take the ferry across the bay (about 7 miles) to Samana if you like. Or, you can feast on freshly caught shrimp at any of the restaurants close to the dock there. Cheap too.
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In El Seybo, one of the neat things to me during the bull fighting time in May, was that on a Sunday, EVERYONE is riding around El Seybo on a horse- hundreds of cowboys. All of the bars parking lots are filled up with tied-off horses waiting for their cowboys (but trust me, there are also lots of pretty nice looking cowGIRLS around too.
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If you go to Lago Enriquillo in the Southwest, ask one of the locals near the lake to show you where to go and see the cave drawings that were made by Taino Indians high in the cliffs above the road, when they sighted some of the first Spaniards checking out their lake below. This is just a little east of Los Rios on the road that passes on the north side of Lago Enriquillo (maybe 5 km from where a big gypsum mine is).
The Indians hid in the cliffs, and I guess to pass time, made lots of these cave drawings there, under overhangs. About 500 years ago. Wear your hiking boots, and take your time climbing up. It is not extremely difficult, but it is unmarked, and it will take you 15 or 20 minutes to get up there. Take a drink along and rest and enjoy the view from above.
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You can take a road trip south of Barahona and go as far as Enriquillo ( a village on the coast south of Barahona). The road is usually quiet, but has many stretches that seem to have a 1000 foot dropoff on the seaside (although NOT along the coastline) of the road, and only a line of little concrete posts keeping anybody from plunging off the cliff. Spectacular views. Watch out for the guaguas and let them pass (as usual)
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Playa Grande, east of Cabarete, when there has been an Atlantic storm, and the 12' waves are rolling in. Best view is from the Playa Grande resort above, on the golf course. If there are no storms, the beach below the golf course is one of the most beautiful as well.
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Want a taste of Dominican culture? Bright and early on a Monday morning, (like 6.30 or 7.00 am) in Santo Domingo, go sit in the shade on the east side of Avenida Maximo Gomez across from the US consulate, and watch the masses of Dominicans standing in line to try and get a US visa, as well as the various entrepreneurs making their living from this whole operation. The show is finished by 11 or so.
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If you are so lucky, see the Faro Colon shining its cross into the sky at night, when it's a bit overcast, so that the cross is reflected from the clouds. Balaguer might have been a turkey building it, but it sure as heck was cheaper than a Metro, and it is a sight to see. That is, if Leonel ever lights it up, other than on Easter.
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Take the road up to Constanza. They have been rebuilding it for years. Narrow, twisting, very few guardrails protecting 500+ foot dropoffs from the gravel road. It is an experience, and on a clear day, you can see 50 or 100 miles out over the Cibao. Same or better at night, or when there is a lightning storm at night over the Cibao. Or, se fue la luz, also a good effect.
There are some great greenhouses on the way to Constanza also, and the people grow flowers and vegetables in its valley. Be careful on the way back down, with the many trucks carrying cabbages, onions, carrots, etc out to Santo Domingo and Santiago.
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Go to Dajabon on a market day (I think that their days are Mondays, Wednesdays or on Friday) and station yourself about 100 feet from the border crossing portal.
What a parade, Haitians carrying in everything they have to sell, and guys with huge carts taking things out of the DR to Haiti that people have come to buy. Customs guys checking things. On the other side, UN peacekeepers with carbines and blue berets watching the passing throngs too. Thousands of people, lots of stuff to pick up as souvenirs, at a great price. And they all leave at sundown, or sooner.
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Good luck.