Isla Saona and Altos de Chavon, maybe a horseback riding and 4 wheel safari in Higuey, Santo Domingo city tour.
Colonial Tours and Travel does excursions, they are a big agency, check their website to have idea about prices. it is not necessary that you use them, but on their website you can check out what's on offer and how much it costs.
Reservation Excursions, Tours Dominican Republic