And if you are on the south coast, get along to the capital for a visit - superb place. You can get a guide for a quick walking tour - worth it to know what you are looking at...!Also , plenty of really, really good places to eat. Lots of cafes and street bars just to sit at and watch the world.
Diving is good - but do check the credentials as DH suggests above. Wreck dives are good if you are near a site - and do take a banana for feeding the fish! Horse trails can be a bit hit and miss though, but try and do the trip to Catalina Island. And do not buy mama juama - silly tourist drink - sticks and compost in a bottle with gin......
Do buy the over-proof rum - dirt cheap and worth every penny...!
Lots of info on hotels at the DR1 site at:
http://dr1.com - enter the destination in resort reports for some background on where you are going.
If you are north coast, get into the central region and do some rafting too. The villages there are really 'wild west' towns and are set in stunning scenery.
Taxis are horrendously maintained - no brakes, broken windshilds, haphazard 'drive in the shade' style - but then the driver buys you an ice cream off the street vendors 'cos its hot and you're stuck in a traffic jam....just the most hospitable people and courteous.
Food is fantastic, beaches good in the main..just say no to the beach hagglers, they do respect your wishes I found while there for a bit - and just chill.
Just one comment to say that you will see some real poverty - it is a fact in the country that they are not in any way well off, and some parts of the country towns are dire, but if you travel with an open mind and respect the people, remember this is an emerging country.
Have a peachy time and enjoy it all..!