Much of Boston is in walking distance. The Freedom Trail is definitely worth doing as it will show you several of the highlights of the city. If you really don't mind walking, you could walk to the park and to the area by the Church of Christ Scientist (great reflecting pool and nice fountain as well as cool architecture). The T isn't too bad, though, and you might look into whether they have a pass that will give you unlimited rides for a certain time period. Back Bay is nice on its own.