Here are two surreal drawing exercises that can be effectively explored in a 75 minute class.
#1. Homage to Andre Breton
Using an exercise handout that has four square grids of equally placed dots, draw and try not to think about what you are drawing. Try to allow the drawing process to be automatic. Do this with the first three grids and then use the forth to refine the composition in a conscious way, using a more usual approach to drawing.
#2. Illustrating a random phrase
Take any passage of text and randomly underline 6-8 words, not in a row, but sporadically throughout the passage. Work out some sketches to illustrate the random phrase. Try this a few times and choose the most interesting one to develop into a larger composition.