Have been there twice and loved it.
If you go stay on the south of the island flying into Punta Cana, rather than the north and Puerto Plata. The beaches and sea are much nice on the south side.
Did some interesting stuff there, and there is loads more to the place than just beaches and All Inc hotels. I think they filmed some of jurassic park there and a trip off into the jungle is a must. We did a jeep safari and saw some local jungle villages, the places where they grow and process coffee, cocoa, tobacco etc. Absolutely stunning.
Also did a day trip to Santo Domingo the capital, was a few hours on a coach either way but again massively worth it. Its the oldest European city in the Americas, with the first street, the first cathedral etc There is a massive lighthouse where they say Columbus is buried, and the house his family lived in when they discovered the island. Also a lot of stuff about the Dominican history, freedom fighters, revolutionaries etc.