One exciting activity me and my students did in my third grade art class is making free form shapes from geometric shapes. I found this very exciting activity in Ms. Kathy Barbro's blog: http://artprojectsforkids.org.
1) short bond paper
2) colored markers
4) brush/ dropper cup
1) Using a pencil, draw different kinds of geometric shapes- circle, triangle, square, rectangle, etc. You can overlap the shapes.
2) After the pencil shapes are drawn, they are to be colored in with a NON-permanent, waterbase marker. This is the time to use those cheap, fat store markers as you actually will be wanting the colors to bleed.
3) Drop several puddles of water on your artwork with a brush or dropper. If they pick up the paper and roll the water around a bit, it should start to make lots of colored streaks and blobs. Repeat this until almost all of the artwork is filled with wiggly colored lines. Let dry.
4) Once it is completely dry, trace all the organic shapes that were made from the running water. You need to work slowly to trace all the wonderful edges you see, both inside and outside the colored shapes. The more time you put into the tracing, and the more detail you see, the better your artwork will look.
Here is an example by Ms. Kathy Barbro: