All you need is paint, some hands and somewhere close by with plenty of soap and water!
A simple idea for Autumn is creating a tree with the leaves made from handprints in autumnal colours, orange, yellow, brown etc. The children could be encouraged to mix the paints to match the colour of collected leaves from perhaps a nature walk before printing their hands. When dry the leaves can be glued onto a trunk, either painted, cut from paper or the children could do some bark rubbings which could be then cut to shape.