This year I am using a sketchbook. Anytime they are finished with their assignments they are to work in their sketchbooks. I give them a list of sketchbook assignments they can do as well as collect pictures, write poetry, etc. It does help if they know that if they finish early they will still need to be doing art. It has taken care of the problem for me and they are enjoying working in their sketchbook. I have battled with the problem at the high school level for years and this has been a blessing. If they "forget" their sketchbook, they can use plain paper and tape it into their sketchbook next time.
It also encourages them to develop some original ideas.
Teresa in el paso