Here's a little sculpture project I like to use - did this today with kindergarten, first and third grade.
It's an inexpensive way to introduce three dimentional art - I link it to a standard on cooperative learning, or some years to one on terms common to the arts and other disciplines.
We look at some modern sculptor's work, talk about abstration and 3d vs 2d. (art that you can walk around.... art that is fun to look at but doesn't look like anything else...)
Then I show some construction tips, shapes that are stronger than others, harp on glue skills, etc, then let them rip.
It's wildly popular - the kids LOVE it, and it uses up all my construction paper scraps from framing pictures for the art show.
These examples are from first grade, 35 minutes. Sometimes we go two days on this, and use whatever fun things I can find like pom poms, feathers and wiggly eyes. This is the cheap and dirty one day version.