You can create your own beach ball with materials that you are likely to already have lying around the house. Before you know it you'll be batting your new ball around with friends.
Grip the trash bag so that the bag below your hand is approximately square.
Inflate the bag by blowing into it until the bag is tight, then gripping tightly to trap the air.
Spin the inflated portion of the bag several times, then tie off the bag at the point you were holding it.
Trim the excess bag from above the knot.
Wrap the bag with the duct tape, being sure to tuck in the corners to create a spherical shape, and to cover the entire exterior with tape.
Paint your ball with water-proof paint.