We did our only RC shore excursion from Boston. It was a four hour tour that went to Lexington, Concord and Harvard. This was very informative and a highlight of our trip. It definitely gives you a sense of history about how the Revolutionary War started. You learn a lot about the first shots of the war. Walking through Harvard's campus was fun, too, although I sensed we were allowed to do so because summer session was going on and not as many students are around. I would recommend the Revolutionary War Experience as a great way to explore the history of the country.
We also walked about a half hour (we are quick walkers) to where the Bay of Fundy meets the St. Johns River and reverses its flow every day. It was fun watching the two bodies of water clash and see a tide go uphill. You can check, like we did at the Visitor Center in town, when this will occur on the day you are in port. I definitely recommend the walk as it culminates in a very scenic bridge lookout over the Bay of Fundy.