Tour of the Harbor and the Neponset Greenway from Castle Island to Paul's Bridge and back. We'll be especially interested in checking out Truman Parkway in Hyde Park.
The Boston Harborwalk passes through Castle Island. Or, take 8th St. from South Boston to Day Blvd. and follow either the road or the parallel path.
You can take your bike on T rapid transit lines and some bus lines for no extra charge. Details are on the MBTA website.
From Andrew Station (Red Line):
Take the JFK exit from Rte. 3 and follow Day Blvd. to the end. Please consider carpooling to the ride.
You can turn around at Pope John Paul II Park or Mattapan Square. The closest T station to most of the ride is Ashmont on the Red Line, but it is a hike.