I don't know if I would call it a journal, but my students are required to observe the moon for a set period of time. They record what they observe each night on a worksheet which I provide. Most of the students continue to observe the moon even after the assignment is over because they find it interesting.
Another fun activity to go along with the phases of the moon....the students use vanilla wafers as the moon, and they dip them in chocolate to model the phases of the moon. (they dip 1/2, 3/4 , etc.).
How about cutting out a hole in a sheet of black paper in the shape of the moon at different phases and gluing wax paper behind it to represent the moon. Then you can make it into a book or poster. They would look cool taped on a window!