A trolley tour as already mentioned. Great for an overall view. They go on a lot about the British. Don't take it personally - they don't make personal attacks, although there may be some gentle ribbing from the drive/guide if you identify yourself as British [English]. When they mention the Boston Massacre - a whole FIVE people killed - just mention that we kill more people on our motorways every day.
If you take one of these tours you'll see a number of things worth a photo. Either make a note on a map and return later or get off at one of the stops and catch another tour bus following.
Beacon Hill district. Old streets, some cobbled, and gas lamps. Great photo shots.
Observation deck. Top of the Prudential tower building for the view.
The Cheers bar was only used for exterior shots. Nothing was ever filmed inside. Overrated.