Arnold Arboretum. The sumptuously landscaped Arnold Arboretum is open all year to walkers, nature lovers, and joggers. Volunteer docents give free walking tours in spring, summer, and fall. 125 Arborway, Jamaica Plain, Boston, Massachusetts, 02130. 617/524–1718;

Boston Harbor Cruises. Take to the seas to see the city from a new vantage point with Boston Harbor Cruises. They offer ferries to the Harbor Islands from Long Wharf (Downtown) with limited service in spring and fall, as well as expanded service in June with additional departures from Hingham and Hull to six island destinations. High-speed catamarans run daily from May through mid-October and cost $19.95 round-trip. Other islands can be reached by the free inter-island water shuttles that depart from Georges Island. The boat touring company also offers leisurely sunset and brunch harbor cruises, as well as narrated historical sightseeing ventures. Thrill seekers love a get-wet trip on their Codzilla, a turbo-charged craft. Long Wharf, Waterfront, Boston, Massachusetts, 02110. 617/227–4321;

Boston Harbor Islands National Recreation Area. Comprising 34 tiny islands and peninsulas, this is one of the city's best hidden gems—and it's literally out of sight. Stretching from South Boston (Castle Island) to the coastlines of South Shore towns Hingham and Hull, each island is different, but most feature abundant nature with miles of lightly traveled trails, shoreline, sea life and wild plants. The focal point of the national park is 39-acre Georges Island and its partially restored pre–Civil War Fort Warren that once held Confederate prisoners. Other islands worth visiting include Peddocks Island, which holds the remains of Ft. Andrews, and Spectacle Island, a popular destination for swimming (with lifeguards). Lovells, Peddocks, Grape, and Bumpkin islands all allow camping with a permit, from late June through Labor Day. Pets and alcohol are not allowed on the Harbor Islands. Ferries shuttle visitors from Boston to Georges and Spectacle islands daily during summer months. Plan to spend a whole day exploring! Boston Harbor Islands National and State Park Welcome Center, 191 W. Atlantic Ave., Waterfront, Boston, Massachusetts, 02110. 617/223–8666; Closed mid-Oct.--mid–May.

Charles River Reservation. Runners, bikers, and in-line skaters crowd the Charles River Reservation at the Esplanade along Storrow Drive, the Memorial Drive Embankment in Cambridge, or any of the smaller and less-busy parks farther upriver. Here you can cheer a crew race, rent a canoe or a kayak, or simply sit on the grass, sharing the shore with packs of hard-jogging university athletes, in-line skaters, moms with strollers, dreamily entwined couples, and intense academics, often talking to themselves as they sort out their intellectual—or perhaps personal—dilemmas. Back Bay, Boston, Massachusetts, 02108. 617/727–4708;

Hatch Memorial Shell. On the Esplanade, the Hatch Memorial Shell hosts free concerts and outdoor events all summer. Esplanade, 47 David G. Mugar Way, Back Bay, Boston, Massachusetts, 02110. 617/277–0365;

Emerald Necklace Conservancy. The six large public parks known as Boston's Emerald Necklace stretch 7 miles from the Back Bay Fens to Franklin Park in Dorchester, and include Arnold Arboretum, Jamaica Pond, Olmsted Park, and the Riverway. The linear parks, designed by master landscape architect Frederick Law Olmsted more than 100 years ago, remain a well-groomed urban masterpiece. 125 The Fenway, The Fenway, Boston, Massachusetts, 02115. 617/522–2700;

Mt. Auburn Cemetery. Cambridge's historic Mt. Auburn Cemetery is known as one of the best birding spots in the area and also has walking paths, gardens, and unique architecture. You can see the graves of such distinguished New Englanders as Oliver Wendell Holmes, Henry Wadsworth Longfellow, and Mary Baker Eddy. 580 Mt. Auburn St., Mt. Auburn, Cambridge, Massachusetts, 02138. 617/547–7105;