Public transport: Boston has buses, trains (commuter rail), a subway and ferries. Get all the info on the official MBTA website if you are based in the USA, or download the MBTA app for iOS, or for Android, to use worldwide.
Bicycles: Bluebikes is a public bike-share program with single ride, daily, monthly and annual passes. Find the nearest Bluebike (and other information) on the official Bluebikes page.
Ferry: sail the sea between Lovejoy Wharf (at North Station) and Fan Pier (at Seaport) with all-day departure times. Visit the ferry website for tickets and schedules.