As Will said, the beach areas of Narragansett, Westerly and Middletown/Newport will be great for the girls - plenty of "beach watching" for them, plus the surf is not large, so there is usually very little danger from riptides. All the beaches have plenty of lifeguards.
If you're girls like shopping, the Providence Place Mall downtown is huge and they can spend the better part of a day there if the weather is lousy.
You can spend easily an entire day in Newport seeing the mansions and doing the Cliff Walk.
Classic New England you can go into CT and spend a full day or two at Mystic Village, Mystic Seaport and the Aquarium.
With the limited time you have, that may be your best bet. Mystic has a number of hotels/motels centrally located - go to www.visitmystic.com
i used to take my girls there about 3-4 times a year until they got older (one is now 17).