This is an easy and very successful project that I typically do with my fifth or sixth graders, but can be adapted for younger grades.
Materials:
Watercolor paper
Watercolor paints/brushes
Coarse salt
Sharpie markers
Large pan of water
Students first dip their paper in a dish of water (make sure their name is penciled on the back first). Place paper onto a construction paper blotter and carry to their seat.
Use a wet on wet technique to add "blobs" of color onto the paper. Teacher should stress that students should not try to make pattern or design, just focus on adding blobs of interesting colors. Sprinkle with salt and observe what happens as the salt absorbs the water and color.
When the paper is totally dry, scrape off salt and begin to add patterns with sharpies. Have students brainstorm different patterns. I usually have the students add them to the whiteboard, so other students can see their interesting ideas. Fill the whole paper with interesting designs and patterns. I usually stress the smaller and fancier the patterns, the more effective the finished picture will be.