Hey, it wouldn't be a Nancy Drew game if there weren't some secret rooms and hidden tunnels, would it? The first hidden place you'll find is the cellar, which is behind the locked barn doors in the tool room.
How do I open the double doors in the tool room?
There are three passages that lead from the cellar; one is obvious, but the other two you'll have to find.
There's also a completely separate house, the place that burned up the night Charlotte died.
There's that locked crypt in the cemetery, of course.
Finally, there's that locked door on the second floor of the main house: