where are you staying? north, south, or eastern end? I did some great diving in the north (sosua and cabrera) if you're interested in scuba. I also did an ATV tour up into the mountains... another cool trip.
the casino's were pretty small on the northern end, but shopping was good and cheap. (just ask my wife). The beaches are incredible. Also If on the northern side, i'd reccomend ocean world? its a marine park where you can interact with the exhibits. i think they have a website. The architecture in the northern cities was excellent.
My advice would be to go to the travel section of barnes and noble, or your local bookstore and grab a travel guide to the dominican. they break it down by region, and have a lot of contact info. And definitely bring your detector. There were several shipwrecks along the northern coast (if that's where your staying), and i was told that occasionally gold and silver coins have washed up on the beach, although i didn't find any
good luck
steve