I can recomment to visit some of the beaches around Las Terrenas and rent a motorbike for a visit to El Limon waterfall which can be reached guided on horseback or by foot from several posts.
The seafront of Samana is also a nice spot and if you still want more go further to Las Galeras with its wide white beach on a nearly empty road...
Las Terrenas is a nice alternative to the All Inclusive type resorts commonly found in DR. It's quite layed back style and proximity to nature are its big draws for me.
It really is one of the prettiest places I have ever been.