Boston is known as a walking city but many people don't realize how friendly it is for cyclists as well.
People living to the west of the city can come into town via the Charles River's well-maintained bike paths. The paths are maintained with striping and everything and offer awesome views of the Charles River and Cambridge on the other side. On many mornings, it is possible to watch the crew teams practicing.
If a bicycle commute isn't what is really desired, a bike through the city can be great too. Many of the city's streets come equipped with bike lanes that are intended for bikes only. And no need to worry about the cars, they're gunning for the pedestrians, not the cyclists! Just kidding. It might be a good idea to watch at intersections for any cars that are completely ignoring the red lights. Also, pedestrians have a tendancy to jump out from between parked cars so any cyclist would be wise to be on the look out for those guys too.
There's also a map that people can use to contribute their own paths. If anyone has time, it's worth looking at. Using your own gmail account can allow you to figure out the mileage of any of your own rides.