Parents and children visiting Martha’s Vineyard flock to Joseph Sylvia State Beach. Waves are minimal and the beach slopes gradually into the water, so kids can play without getting in over their heads. The beach stretches about 2 miles between Oak Bluffs and Edgartown, and there’s parking along the road that connects the two — just try to arrive before 10 a.m. on a hot summer day to get a spot. Of all the swimming beaches on the Cape Cod National Seashore, Coast Guard Beach in Eastham (lot parking available) is the most popular with families. There’s plenty of room to spread out, even on the busiest day.