Southeast of Boston and Hingham, the 3,500 acre Wompatuck State Park (aka Wompy) has tons of mountain bike singletrack blazed in orange, plus a 12 mile paved pathway. The rocky and rooty trails roll gently through maple and oak forests.
To reach Wompatuck State Park from Boston, follow SR3 south to Exit 14. Turn left heading north on SR228 for 3.3 miles. Turn right on Free Street for 0.8 miles to the park entrance on the right. A nice place to park is right at the visitor center.
Overall, Wompatuck State Park is the 2nd most popular mountain bike trail of all 35 mountain biking rides in Massachusetts.