First Graders love the Magic Treehouse series by Mary Pope Osborne.
For this project, students started by coloring their background with oil pastel: the tree, the sky, the ground. They choose all the colors, the tree type and size. They can also create whatever house shape they want to . Most things we do in the art room are like this. I throw out the idea, show them how to use the tools and they run with it.
We talk about what things you might see in a treehouse-rope ladder, tire swing, door, window, balcony, etc. Once the background is in, they create the house and details with collage.
I have a giant box of scraps-mostly sample paintings that I have chopped up or scrap donations from other teachers.
Here are a few finished treehouses. I love how each one is a little bit different.