Stay in SD for a week and take in the city. The best variety of night life, restaurants and cultural activities in the country.
Go to Santiago for a week and enjoy city life and a small town atmosphere all in one. Great people, beautiful women, and some cultural activities, though not as many as SD. Go to Jarabacoa for a day or two and enjoy the beautiful nature, rivers, and pleasant cool temperatures. Lots of beautiful and friendly women too!
Go to Cabarete for a week and enjoy the beach. Very good night life with a great mix of locals and travelers from around the world. Sosua and Puerto Plata are a short car ride away, so you have some more options during your stay.
Finally, I suggest a week in the south. It's not as tourist friendly, but you'll get to explore some of the most beautiful and unspoiled places in the entire country. Start in SD and make Bani your first stop. Go to the sand dunes, get your fill on some of the best mangoes in the world, and see some of the prettiest women in the country in this quaint little town. Next stop is Barahona, but a little rest stop in Azua for some great chivo is recommended. Stay in Barahona for a few days and enjoy the pebble beaches (San Rafael, El Quemaito - not much sand) and the river next to it. Take a trip to Bahia de la Aguilas for the most spectacular beach on the entire island. Keep heading west and go to Lago Enriquillo and see the iguanas, and if you have time and want to see the salt water crocs that live in the lake, take the boat ride to Isla Cabritos. Go to Parque Nacional Sierra de Bahoruco and take in the beautiful nature, see the larimar mines and experience the unique feeling of being around sacred native indian grounds.
There's more to see and experience, but I think that this gives you a good start.
