Here are 2 sites with GREAT lesson ideas on them.
The first is
The second is from an Elementary school in Michigan. the art teacher their has some fabulous lesson ideas. I hope she doesn't mind me sharing her site!
Here are two more great art sites that can be used as part of your teaching unit:
A kind of educational 'facebook' where, students can post comments, papers, or answers to assignments. I feel it would really be more beneficial for upper levels but it is possible to use with 4th and 5th graders.
A GREAT site where you can start a wall and have students post 'sticky's' on it about any topic you want. The way I do it is by posting images and links to websites (child-appropriate ones) and then have them post facts, opinions, or ideas onto our wall. It's a great tool for collaboration too!