I agree, this can be a significant challenge. Short courses encourage less "true" efforts and an over-focus (for lack of a better term) on outcomes (grades). Students may become grade oriented as opposed to knowledge oriented. This often results in cramming and short term memorization. However, I think that the structure of the course is pivotal in maximizing the overall learning in a short course. Perhaps the course should contain more hands-on experiments or student projects as opposed to papers and tests about textbook content. I think that offering alternatives to tests and papers may motivate students to appreciate the actual "learning" process, not simply the resulting grade. Any thoughts?
