If staying in Edgartown or in the Mattakesett area, a nice run to simply uncork is to enjoy the bike paths along Katama Rd. Let the rest of the family schlep the beach gear and deal with parking and run the 2.6 miles from Edgartown to South Beach, along Katama Rd., with a bike path all the way. It’s open, so be aware of sun on a hot day.
For a full 6-mile loop, once at South Beach, head along Atlantic Dr., through South Beach State Park for about 1 mile, and then head back to Edgartown using Herring Creek Road. This road isn’t all that interesting or scenic, but there’s a bike path the whole way.