Lucky for you guys I just sorted through a few years of photos with the extra time..
A few places in Andover:
The Vale Reservation is right off the Ballardvale stop. The entrance is at the end of the parking lot. Any time of day, any season, you'll probably see something.
I've chased a number of blue herons and other birds in here.
For part of the reservation, you walk along the Shawsheen River as it runs along Pomps Pond, which sits about 5 feet higher than the river. Kind of interesting to walk along it. Pomps also has walking trails, a private Girls Scout Camp, and a beach primarily for children (membership or payment required), though its less kept up compared to everything else on this list to me. Still a nice hike.
Winter a few years ago:
Goldsmith Woodlands is a fairly long hike with several 'points' around Foster Pond in Andover. Some nice sights, buggy in summer.
A foggy shot from last March:
A personal favorite: Baker's Meadow Pond - a great spot (especially if you're into migrating ducks)
2 days ago:
Final Andover spot: The Cochran Wildlife Sanctuary, a large preservation behind Phillips Andover that was protected to provide students a space away from the campus. The rationale behind the space was beautifully put. I'll try to find the discussions of the architects/planners for the sanctuary. In the end, they found high school boys were not interested in walking around a forest, but it remains in place today. I don't have pictures, but there is a log cabin still in use today for larger functions. It was originally intended to be a hangout spot and dining hall for students during their free time.
Remnants of what (if I recall correctly) was once intended to be a clubhouse spot for the Phillips students. I think it then became a maintenance storage building, and now sits in its current state: