This 281-acre living laboratory is somewhat incongruously set in a dense urban area. Established in 1872 in accordance with the terms of a bequest from New Bedford merchant James Arnold, it contains more than 4,000 kinds of woody plants, most from the hardy north temperate zone. It's a mecca for runners, cyclists, dog-walkers, and families. The rhododendrons, azaleas, lilacs, magnolias, and fruit trees are eye-popping when in bloom, and something is always in season from early April through September. In October the park puts on a display in blazing colors. Peters Hill has a grand view of the Boston skyline and local surroundings. The Larz Anderson bonsai collection contains individual specimens imported from Japan that are more than 200 years old. In the visitor center there is a 40-to-1 scale model of the arboretum (with 4,000 tiny trees), plus rotating exhibits. If you visit during May, Lilac Sunday on Mother's Day is an annual celebration of blooming trees, family-friendly activities, and picnicking (the only day it's allowed). The arboretum, 6 miles from downtown Boston, is accessible by the MBTA Orange Line or Bus 39 from Copley Square to the Custer Street stop in Jamaica Plain (3 blocks away).