In terms of places to jog, run, rollerblade, or simply walk the dog, the Esplanade along the Charles River is a popular spot. Along the Esplanade, you will also find the Community Boating House where members can rent boats to sail in the Charles.

The swimming pool at the MDC at Cleveland Circle is open during the summer, and in winter, the MDC opens its ice rink to the public for a small price.

The Frog Pond in Boston Common is but a wading pool for kids during the summer, but it turns into an outdoor skating rink for those who want to fully experience the charm of Boston in the winter.

At the end of the Orange Line at Forest Hills, you will find the Arnold Arboretum, a massive plot of land owned by Harvard University that is filled with trees from around the world.  Be sure to stop and check out their bonsai collection.  Those looking for a short hike near the city will be satisfied.  Anyone looking for more serious hiking will have to drive to the Blue Hills. 

Those who venture into South Boston will find miles of beaches and waterfront parks, even if they may not be the cleanest or most impressive that you've seen. Castle Island, however, is easily accessible by public transportation and is a great place for a stroll or a swim.

In the summer there are plenty of great beaches merely a train ride away from the center of Boston.  A picturesque location on the beautiful north shore is Singing Beach in Manchester by the Sea.  Simply take the Newburyport commuter rail line and in about an hour you'll arrive within walking distance from beautiful views of the Atlantic.  If you're in Boston with a car for a longer period of time, you can't go wrong with a visit to Cape Cod or the islands (Martha's Vineyard or Nantucket).

If sitting on the beach doesn't offer enough activity for the active types, another option is kayaking or canoeing on the Charles River.  If you have a car, a quiet option is to venture out along Comm Ave until you reach the 128 intersection.  A boat house sits along the edge of the river right before you approach the highway.  For a nominal fee, you can rent a boat by the hour and within minutes you'll be paddling along quiet waterways without the feeling of being anywhere near the city. Great for a beautiful sunny day in the summer or fall months.