Joe, I teach middle school and we have done altered books in a number
of different ways. I have found this to be very motivating. One
suggestion is to have them tear out a page and make a media "chart" of
wet and dry media...like columns with oil pastel, charcoal, etc
crossed by rows of oil pastel, charcoal, watercolor, etc.
Although I don't think it is overkill as you asked, why not combine
the two (sketchbook and altered book)?
My student teacher this past year gave students prompts to choose from
and treated them kind of like a visual journal. Even if they had also
chosen a theme they would keep that in mind when exploring the
prompt. One choice was "Think of a recent dream you had. What
emotion did you feel during or after you woke up? Alter the page to
describe your dream and the strong emotion you felt."