I would recommend Great Brook Farm State Park in Carlisle, lots of neat trails and a home-made ice cream stand in the summer time.
(not sure if its open yet)
Stop at Fern's Country store on the way or after.
its cool too
Great Brook Farm State Park
$2 to park for a Car
Trail Map:
http://www.mass.gov/dcr/parks/trails/great-brook.gif (broken link)
Fern's Country Store, Carlisle
Ferns Country Store - Home
Great Brook Farm State Park in Carlisle - FarmFresh.org
Hope this helps :-)
there's also in Bedford ...at the beginning of the Minuteman Rail Trail there's
another trail right across Loomis St. from the Parking area another trail (next to the pizza /sub shop)
That goes north, into North Bedford, and Billerica
(Narrow Gauge Rail Trail)
Hope this helps
Sincerely,
jeff D
Winchester, MA