Collage
Refill
Find an image with a lot of detail but an obvious central figure or some kind. Cut that figure out of the image (and save it for a loter project, of course). Now, using either another image as the background, something you paint, or other collage, place your negative space top sheet over it.
Redo
Find a piece you never finished or didn't like the end result of and use it as your base to collage over with other materials. You could follow its lines and colours, or you could cut that old piece up and use it as collage materials.
Exquisite roll
Using a scroll of paper (or sheets attached to each other), start a collage of some kind and make maybe six inches of it down the roll (or however much you like). When done, roll the scroll up leaving on a sliver of the work visible for the next person to work off of. When they're done, do the same. Repeat until everyone participating has had a chance to add to the piece. Unroll!
Re-shape
Using a single image (something from a magazine perhaps), cut or tear that image into shapes or strips and rearrange them into a new abstract final product.
Collective
Gather together multiple images from magazines, newspapers, junk mail, and whatever else you can access, and use the colourful pieces as "paint" to create another image by cutting the source materials up into appropriate sizes and shapes of colour.