Here are two ways to create wonderful fall leaf decorations and art in your classroom. I did both of these with preschoolers, but they can be done by all ages and are very satisfying.
The first project involves coffee filters and markers. I know you've done this before, but they came out so well I had to mention it here. We used the extra-large, extra-thick ones from Discount School Supply, but regular ones will work as well. First, cut out various leaf shapes from the filters. Try to get a few different species so you have a variety. Then, have the kids color the leaves with markers. I limited them to red, orange and yellow, although you don't have to. Show them how to make dots or stripes on the leaves, as coloring the whole leaf in is unnecessary. Now just have them drip water on the leaves using pipettes. Don't let them put the whole leaves in water, or the color will leach out. Lay them on sheets of paper to dry and you'll have gorgeous translucent leaves that can be hung or taped to windows.
The second project is a classic, and with good reason. Collect real leaves, apply paint or ink to them, and press them against paper. Even using cheap children's tempera, you'll get wonderfully detailed veins. I liked using sponge rollers instead of brushes to apply the paint; brushes lay it on too thick and you get a globby mess. Also, place another piece of paper on top of the leaves and press on that rather than directly on them. This stops the leaves from shifting and creating blurry images. Finally, I suggest using plain white paper as your background. A few of my co-workers did the project with brown or green construction paper, and the detail just didn't show up as well as it did with white.