We're coming over in April for 4 nights and we'll be based in Charlestown. We'd like to spend one day in Salem, Gloucester and Rockport. As we won't have a car, we'll have to go by train. The train schedule looks as though it's feasible, but are the main tourist attractions all within easy walking distance of the train stations? Is there a day ticket we could buy or do we have to get separate tickets? Any recommendations for 'must do' attractions in each place? Thank you!