Greenfield State Park, Greenfield, New Hampshire Greenfield State Park is a 400-acre public recreation area in Greenfield, New Hampshire. A network of walking and mountain biking trails surrounds the Greenfield State Park campground area. The most popular trail is Hogback Pond Loop, which has a 4.5 star rating and is 1.4 miles long with an elevation gain of 88 feet. The hiking and biking trails feature bogs, forests, and the shoreline of Otter Lake. Trails lead to Beaver Pond, Mud Pond, Hogback Pond, and Spruce Swamp, withthe Hogback Pond Trail being relatively flat. The park is ideal for multiple activities beyond hiking—activities include camping, hiking, swimming, fishing, picnicking, and non-motorized boating. Pets are permitted on-leash in the campground and on the trails.