This will be the first year in awhile that I will be reintroducing sketchbooks, and only with my advanced class. I used to do it with my beginning class and found out very quickly that the sketches deteriorated into them doing it 5 mins. before class. The great art kids in those classes did their assignments (but that would only be 6 out of 18 kids), and the rest would dash them off. I found that very frustrating since the great kids would normally be sketching on their own anyway. So what I did was institute the idea of thumbnail roughs for all assignments, and that would take care of the sketching exercises.
However, when I DID do sketchbooks, I would give them a list of assignments and when they were due. This list would be taped into the front page of their sketchbook. I also had my assignments mirror what we were doing in class. So, if we were doing line first (this is the beginning class) their assignment would be line driven, etc.
With the advanced students their sketchbooks will be idea driven, and will also emphasize drawing from observation, two skills that I push in class. Over the years I have had students' sketchbooks be the entrance into art colleges along with their portfolios. Colleges are eager to get a peak inside students' artistic, creative minds, and would rather see little experiements and ideas that are generated in their sketchbooks than finished, tight, "project driven" work. I think I read somewhere many years ago that Bill Gates owns one of Leonardo's codexes (or is it codicis?). Anyway Leonardo's sketchbooks gives us a window into what he was fascinated with, and I think along those lines I try to have assignments that allow students to "ponder".