If Dreams are anything like the Iberostar resorts there will be plenty to do, especially if you hang out around the pool where the entertainment staff works hard to keep you busy all day.
Having said that, I would advise that you take at least 1 or 2 trips off the resort for a change of pace to see some of the local scenery. If you are interested in some great snorkeling I advise visiting Xel Ha (
http://www.xelha.com/ ) this is a terrific snorkeling area where you can easily spend the whole day. There are also some interesting early Mayan sites in the area. Take one of the packaged tours and you will have a good time. Have fun!