Do a Duckboat tour.
Just walk around downtown. Boston Common, Faneuil Hall. If you're into history, walk the Freedom Trail, see the USS Constitution.
Eat in the North End.
If you've got time, pick up some cheap tickets to Fenway. Check out the.Red statue on a bench outside Quincy Market, and Russell outside of the Garden.
I can't help you with the chowder. If you want a great sandwich, I recommend Sam Legrasse's. They're not open on weekends though.