Santo Domingo, the capital has a lot of interest to see: Colombus' House, the old monasteries and churches, the museums, the Faro a Colon. I'd say you can see most of the main attractions in a week. The North Coast is certainly nice: Rio San Juan, Sosua, Cabarete, Las Terrenas, and SE of SD, Boca Chica and Juan Dolio also have nice beaches. In the interior there are mountains. Check around this Forum and perhaps get a good guidebook. It's not a big country, but you could spend years traveling around and still not see it all.