Holly Shores is an easy trip to both Cape May and the Wildwoods. In August, it would be smart to have reservations all along the coast. I'll second taking the Cape May - Lewes Ferry across Delaware Bay.
From there you can head right down 113 into 13 and over to Assateague. When we stayed at Assateague, we were on the bay side. The wind was pretty rough on the ocean side. Again, if you can make a reservation, do so as early as you can.
There is no easy way from RI through NJ. I'd suggest 95 to 287 and over the Tappan Zee Bridge and then right onto the Garden State Parkway.
If you are coming home on I81, Western Village in Carlisle, PA just west of Harrisburg isn't a bad stop and probably in the area of 6 hours out. Just check that isn't the August weekend for the Corvette Show in Carlisle.
