Boston and Cambridge are rich with state and local parks with fun activities all year

Thirty-four islands are located an easy ferry ride from downtown Boston comprise the Boston Harbor Islands National Park Area. Great for day trips, although camping is allowed on four islands (reservations are required). Two islands have great swimming beaches; one is lifeguarded. Wonderful for walking, picnicking, swimming, and fishing. Find a ferry to the islands.
Season & Hours: Mid-May to mid-October; see ferry schedule
Learn about camping, hiking, fishing, swimming, and attractions at this park.

This park is located along the three large, north-south lobes of Cochituate Lake. The northern and middle sections of the lake are divided by the east-west Interstate 90 and State Route 30. The middle and southern sections are split by the east-west State Route 9.
Activities include boating, swimming, and fishing on its three large lakes. Picnicking, swimming, and boat launching are limited to the middle lake, where the main body of the park is located. Boaters may gain access to the other lakes through channels under roadways. Jet skis are not allowed.
On heavy-use weekends, the parking area can reach capacity, requiring temporary closing of the parking lot. For closure updates, follow Twitter at @MassDCR or call the state Department of Conservation and Recreation at 617.626.4973.
Daily parking fee: $8 for Massachusetts vehicle; $10 for out-of-state vehicle

This urban park consists of a three-mile stretch of parks and beaches along Dorchester Bay, easily accessible from Boston. Carson Beach offers some beautiful views and public amenities. Carson Beach also has a walkway for people to walk, bike, or run along the water's edge from Castle Island to the Kennedy Library.
Season & Hours: Open year-round.
Learn about hiking and fishing at this park.

Charles River Reservation is a park extending from Boston Harbor up Charles River for 20 miles. It touches recreational properties and parks in Boston, Cambridge, Watertown, Waltham, Newton, Weston, Needham, Wellesley, and Dover. Recreation in the reservation includes walking, bicycling, bird watching and water sports like rowing and paddling.
The part of the reservation from downtown Boston is the Charles River Basin, which includes the Esplanade on the Boston side. The esplanade is the location of the July Fourth concert. The Upper Charles River section begins at Watertown Square and extends to Riverdale Park in West Roxbury.

Map of Charles River Basin (PDF)

The Mystic River Reservation is a string of parks and green spaces along the Mystic River and including the twin Mystic Lakes. The two major lobes of the Reservation are along Mystic Valley Parkway (Route 16) but are divided by one offshoot of the Mystic River and the Fellsway (see map). Activities at these parks include walking and jogging, bicycling, picnicking, and access to water sports at Mary O’Malley Park and the Mystic Lakes.
The Reservation is accessible by commuter trains. Take the MBTA to W. Medford or Wedgemere stations for access to Mystic Lakes. Take the Orange line to Wellington Station for access to Mystic River. Bus lines to Medford Square provide access to the bike path.
Open year round, dawn to dusk.