How about this for a cool way for kids to practice shading to create the illusion of depth?! You could start by having your class collect their own rocks/pebbles to draw, or you could use a photo of rocks, like this photo of river rocks taken on my vacation in Montana this summer. I ask kids to imagine what it would look like if each of these rocks had a magical patterned stripe (or stripes) around them!
This can be done with any number of rocks appropriate for your age group or time restrictions. It starts with a pencil sketch using a light source to identify shadows. I know I have written about my light source icons, but it bears repeating. Here is a link to that lesson idea: http://kids-finelines.blogspot.com/2010/10/sunny-day-great-shadows.html I find that often students need practice in how to press harder to get darkest values and lighter for highlights.
Once the pencil drawing is complete, students create bands of color patterns using permanent (or watercolor) markers. I like Sharpie fine points for this. I think that this is a good way to review (or teach) color elements at the beginning of the school year ( primary, secondary, complementary, etc.) This would also be the time to point out that the color bands will be curved, following the contour of the rock in order to appear more realistic.
The rocks can be cut out and then stacked, overlapping on a background paper like in the photo below. You could easily turn this into a collaborative project by having students make individual rocks, cut them out and glue them as a class cairn (or rock pile).
For those with more advanced drawing skills, the rocks can be drawn stacked with overlapping parts and then colored like this:
I like using the book, Three Pebbles and a Song by Eileen Spinelli, as a literature connection for this rock project.
As I look at some of these rock compositions, I see that you could do essentially the same lesson in the spring with eggs as the subject.
Rock on, artists!!