Every year in my preschool class we watercolor Roylco Color Diffusing Leaves. The leaves come in four varieties Maple, Sweet Gum, Oak and Sycamore. Usually after the children apply watercolors we offer them a little spray bottle of water to mist on the leaves -- the extra water helps blend all the colors. This craft is great for ages 1-99. You can also use eye droppers and food coloring if you don't have watercolors.
These leaves look amazing hanging in a window. I couldn't help but purchase some for our family to do at home. Alternatively, you can simply cut out leaf shapes from coffe filters for a similar effect. Here is a pic of my 4.5 year old's leaf: