If the weather's nice, plan a walk. Start at Park Street Station, take a loop through part of Beacon Hill, then through the Common and Public Garden. That puts you right on the edge of the Back Bay. Newbury Street is where a lot of the posh shops are, Commonwealth Ave will have some of the grander 19th-century houses. Decent dining options will be thick on the ground around there. Copley Square is a pleasant destination just a block over from Newbury.