Martha's Vineyard is a great day trip. Catch the ferry from Woods Hole to Vineyard Haven or Oak Bluffs. Nantucket is a longer adventure, catching the ferry from Hyannis. You can bring a car, hop on the bus, or rent a bike or moped to explore the islands.
Want to see the Cape Cod Canal? Hop on a canal cruise to see it from the water. Alternatively, there are paved walking paths on both sides.