The southwest portion of the island is beautifull with some very interesting scenary. Driving form Santo Domingo you will cross the provinces of Azua and Bani. The area i very dessertic, but beautiful anyway. Bani has the largest sand dunes in the caribbean and a wonderfull beach for windsurfing called Salinas.
If you keep going west you will arrive in Barahona, which has beautifull beaches of pebbles rather than sand called Los Patos and El Quemaito. The San Rafael River is also worth a visit if you're in the area, as it is the town of Polo.
If you go to Santiago make sure you visit the E. Leon Jimenes cigar manufacturing plant. This is the oldest cigar producing company in the island and they offer a very nice tour.
I normally go for only 4-5 days at a time. I have some friends/family in the capital. Do you think I would have enough time to go to Lago Enriquillo and back in a day? or better to go
to Pedernales y Bani and on the way back hit L. Enriquillo?