A few weekends ago, I went to George's Island. One of the 34 Boston Harbor Islands that you can take the Harbor Ferries to.
The view that you get from the ferry boat of the Boston Skyline is incredible. It also helped that it was a beautiful clear day.
The Island was fortified by the French during the American Revolution in 1833. In 1862 the fort was used as a prison for Confederate soldiers. The fort was decommissioned in 1951. 10 years later it was purchased by the Commonwealth of Massachusetts and opened to the public.