Chalk, child-size tennis rackets, Nerf tennis ball
You need a driveway, chalk, rackets and a Nerf tennis ball. The net can take multiple forms -- an actual net; 3 milk crates and some two-by-fours; or a long piece of rope. Adjust the net according to the child's age and height. Basic tennis rules apply. Make the court long enough for ground strokes, though most of it will be net play. Doubles make it even more fun.