Kids love to draw and love to show off what they've made. You can combine the two by making t-shirts with the kids' drawings on them. This can be done with markers or with acrylic paint. The paint will be messier but is more versatile for fields of color. Be sure to use markers and paints that are laundry safe so the artwork will last through the wash, and not ruin other clothing. Kids should also be told that this activity is only for these shirts, not any clothing they may have.
Children can all bring several weeks' worth of recycling to their art class. With these jars, tins, egg cartons, and pizza boxes, all kinds of interesting sculpture can be made. Don't let the humbleness of the materials make you lose sight of the lessons that can be taught. A pile of cans can teach a child about mass, perspective, and composition just as well as a bronze statue. Using tape, cans can be made into cylinders, or secured side by side, into hexagons made up of circles. There are many opportunities for teaching not only about art but about geometry, math, and physics as well. Be sure not to use any cans that may have sharp edges.
Several boxes of chalk and a large flat surface are all you need to get the kids started on a chalk mural. Help them to plan it out on paper beforehand, so there is some composition to the final product and not just random scrawls. Teach the children about things such as the progression of events in a mural, the interaction of close and distant figures, and the possibilities of integrated text, insets, and different frames for different events. Presenting a mural as a giant comic book will make some kids more interested.
Kids love this activity, and it's free. If you have a large grassed area, wait until it has been mowed, or until the fall when there are piles of leaves. Give the kids rakes, and rake the grass clippings and leaves into giant patterns. This gives the kids a chance to work together, and to make the biggest art they've ever made. You can make abstract designs across the field, or try to make a picture of something.