Near North Andover, the 3,500 acre Harold Parker State Forest has miles of trails and 35 miles of logging roads open to mountain biking, hiking and horseback riding. The trails roll through maple and oak forest, low lying swampy areas, and rock outcropings. This forest can be quite moist and lush during the summer producing a bumper crop of mosquitos - so bring lots of repellant.
To reach the forest from Boston, follow SR93 north to exit 41, follow 125 north (toward Andover), for about 2.5 miles to the State Police Barracks (on right). Turn right on Harold Parker Road for 1.2 miles to Jenkins Road Turn left on Jenkins for 1 mile and then right on Middleton for 1.2 miles and look for Headquarters parking area on the left. While you can park at the HQ, there are many other nice trailhead parking areas along this route.
Overall, Harold Parker State Forest is the 5th most popular horse trail of all 23 horseback rides in Massachusetts.