Have students sketch what they see quickly in the form of simple scribbles. Place objects at the front of the room and have them sketch them in 10 seconds or less in graphite or charcoal. Then move on to people. Have a student stand at the front of the room and pose, as the others draw them in scribbles for 20 seconds or less. Then have the student move on the spot as the others capture the movement in the same scribble form. This gets students used to drawing what they see without having time to question it and gets them used to capturing movement freely.
Take students on a fantasy journey as they draw using their imagination. Have students close their eyes and, even lie down if necessary. Next, tell them a story that incorporates shapes periodically. As you discuss each shape, have students "draw" them in their mind. Encourage them to imagine using various mediums and textures, drawing each shape multiple times and not limiting themselves solely to materials that they have used before in reality. Imagining drawing shapes and forms helps greatly when the time comes to translate them onto a paper or canvas.