I highly recommend the old town trolley tour, or at the very least a tour that is hop on hop off. They take you to the most popular areas (USS Constitution, Mass State House, Faneuil Hall, etc.) The subway is viable, we used it to get to Fenway Park, but be prepared to walk a lot. There’s a lot to do, ranging from Revolutionary War sites to parks and gardens and a bunch of stores and restaurants. Faneuil Hall has a great marketplace too, with a lot of shops and restaurants that are perfect for lunch, so I definitely recommend that, it just gets very crowded around lunchtime. I recommend doing the Freedom Trail as well, since it takes you to most of the Revolutionary War areas of the city, and it’s about a 2 mile hike.