the big ones have been listed, and I absolutely agree. the museum that Rich mentioned in North East is the Upper Bay Museum, which Vernon Bryant has been instrumental in revitalizing. the hours are quirky, but i highly recommend it....not as big as Havre de Grace, but a short drive from it, so you can do both in the same morning. some really great boats, a massive collection of really cool v-boards, etc. if you want some recommendations on places to eat, let me know...i'm better at food than ducks.
Frank