We have been there 4 times already and are booked for Feb. 13 - 28, 2002.
If you are agile enough there are things called "Jeep Safaris" which take you into the interior a bit and see a lot of local colour and meet with locals.
We took a bus trip to Samana. Just to see a different part of the country, and if the timing is right maybe get in some whale watching.
You didn't mention where you were staying but if it is at one of the all-inclusives at Playa Dorada by all means take the tour of the Puerto Plata and Sosua which is on offer.
We never did go, partly because my wife is not happy with heights (unless she's in an airplane) and partly because it was broken until about this time last year, but you may enjoy the ride up the mountain - and for the lfe of me I can't remember the name of the apparatus at this moment.
Also you should probably try to take the tour of Santo Domingo. It's an all day excursion - and by all means stick close to your guide. There are many interesting things to see in SD.
There are probably more that you will find when you get there.
Enjoy