For Tewksbury Residents looking for a great place to walk on maintained trails, mountain bike, or walk pets -- the AVIS trails along the Merrimack River are wonderful. http://www.avisandover.org/deer_jump.html
Access to the trail on the Tewksbury end is just down by the water treatment plant off of River Road. This part of the trail goes down behind St. Francis.
There are many access points along the nighborhoods on River Road. I find the best parking to be on Launching Road and Burndrett Avenue. You can also access the field trails from the down near the Residence Inn and Springhill Suites in Andover, with plenty of parking.
It's quiet down there, and beautiful! Not many people know about it, and although I go often, I hardly ever see anyone. The trails are skied on and walked on in the winter, too... so it can be year-round fun!