The only art project I recall isn't really traditional art, but it made a lasting impression on me. We used light-sensitive paper and went outside (which I love) and gathered leaves/sticks/rocks. Then we put the object of choice on the light-sensitive paper and set it in the sun. The paper around the object changed colors. We removed the object and the "print" remained. I thought (and still think) that is the coolest thing!
I am a volunteer art teacher at DD's school. We use a formal art program and teach the same 8 lessons each year: line, shape, value, space, color, texture, composition, and famous artist. The specific artwork is different for each grade level, becoming more sophisticated as the students gain the necessary skills. I've done this for four years now and
it! There are so many great art lessons for these basic principles and we always emphasize at least one famous artist at the beginning of each lesson. I'm finishing up third grade next month and the students are really getting it.