When I need a simply sublime experience of the outdoors, I head for one of Mass Audubon's sanctuaries. Mass Audubon sanctuaries are masterpieces of land management. The sanctuaries provide extensive, protected wildlife habitat, wonderful passive recreation (walking trails), and extensive educational programs.
I'm a big fan of land conservation groups in general, such as the Trustees of Reservations (TTOR), but Audubon is one of my favorites. The three sanctuaries that I know the best are Drumlin Farm in Lincoln, Wellfleet Bay, and Broadmoor in South Natick.