I too have done the monster truck tour and would highly recommened it, I did the same as Greg but ended the day boogie boarding near Cabarete. It was a fantastic day out and extremely interesting to meet the locals who were more than happy to see us. In the end everyone was quite drunk as they feed you lots of rum on the way.
One note though is some reps tell you to take sweets to give to the local children on the way but that is not advised as the sweets rot their teeth and there are very few dentists around. If you wish to take something with you, for the children, colouring books and pens/pencils are better and lets be honest really cheap to buy and way more fun for the kids rather than a 1 minute sugar hit!
While in the Dominican I would also try the tour of Cayo Paraiso - a fantastic sand bank miles out to sea with great snorkelling opertunities. Horse riding is also popular on the island but make sure the firm are reputable the first time i went treking there it was only me and the guide on horses which looked close to death.
Once there im sure you will find loads to do and actually fitting a day to lounge on the beach will be the difficult part.
