Plan to be gone for most or all of the day while visiting Martha's Vineyard, Nantucket, Boston, Newport, or the Outer and Upper Cape (Provincetown, Orleans, Chatham, and Brewster).
Venture away from 55, but not so far that you'll be gone all day while visiting other towns Mid or Upper Cape (Sandwich, Yarmouth-Dennis, Hyannis).
Stay close to 55 and check out what Mashpee or its closest neighbor (Falmouth, including Woods Hole) have to offer.