Many trails run through the Fells, many of those under tall forests.
The Fellsway is the road through the Fells West, and is a lovely, though somewhat high speed road.
The Skyline Trail is a major attraction in the Fells.
The view to Boston from Wright's Tower is a popular, for good reason.
Most of the lakes and ponds are water supply, but also supply terrific views.
There are many large stone outcroppings along with "glacial erratic" boulders.
There are many trails, but most are well marked.
Although cut in two by Route 93, both the East and West parts of the Middlesex Fells provide miles of great hiking and biking.
On perfect weekend days parking along the Fellsway can be a bit tricky, but it’s worth the effort to get there and hike.
Commonwealth of Massachusetts
Official State Page for the Fells, including rules and regs.
Friends of the Fells
An important citizen group that helps to keep the quality of visits to the Fells up to par.
Wikipedia
Article about this pair of sister parks.