I used to live and work in Sandwich for a time in my early 20s. Like Doug, I also LOVE the Atlantic White Cedar Swamp Trail (it is absolutely magical) as well as the trail at Fort Hill site in Eastham and at the National Seashore Salt Pond Visitor Center there. If you go as far as Brewster the trails at the Cape Cod Musuem of Natural History on 6A are fantastic and Nickerson State Park is just down the road, too. Right in SAndwich there is also Sandwich Town Beach, which is hardly a walk at all but the boardwalk through the dunes is really lovely and there is some rocky beach/tidepools due to the currents here on the other side of the canal. Have fun!