Stop by National Park Service Visitor Center for some useful tips before you set out exploring. It's an one-stop shop for travel information.
You'll be spoiled for sightseeing choice when staying in Charlestown. National Park Service Visitor Center is an ideal place to kick off here, but there are loads of other worthwhile places to appreciate. Fenway Park, Boston Common, and Faneuil Hall Marketplace are not far from the tourist information center and will fill in your day nicely.