Depending on the student and how advanced he is, I would recommend limiting how many cubes he rolls (there are a total of nine). I would probably have him start off with one. Have him roll the cube, and then have him brainstorm what the picture makes him think of. Now, ask him to think about how he would express those thoughts in music. Have him start with a few notes - probably some type of melody line - and expand from there as is appropriate to his skill.
You could even use this as part of his weekly assignment. You could have him roll the cube and write down what the picture is, so that he can come up with a composition idea over the week.
If you teach a group class, you could also use these cubes with a group of students.
Those are just some ideas! What ways do you see yourself using the cubes?
Disclaimer: These recommendations are purely my own based upon what I personally have found helpful.