Prepare your ball. Hose down the ball, scrubbing off any caked-on gunk and detritus that may have filled the finger holes. Use warm, soapy water if the ball is extraordinarily dusty or dirty. Dry well with a soft cloth.
Examine the ball for inspiration. Look at the ball and note any etchings, cracks, dents or other markings on the ball that make for a great centerpiece, nose, eyes, outline of an animal or whatever else may come to mind. Even chipped balls can make great ornaments as they give you a place to start your design.
Think of things that are round. Another way to come up with a design is to play off the shape of the ball. A bowling ball, especially with its three holes for two fingers and a thumb, works well as a person's head. You can also make it into a globe, a rolled-up slug, a ladybug or any other roundish object.
Brainstorm a theme. If you are still not having success figuring out what you want to do with the ball, think of the effect you want the ornament to have. Cozy could end up being a teddy bear. Funky might be a swirly ball with stars all over it. Patriotic might mean painting a rounded flag on the ball. Scary could be a misshaped gnome or troll.
Decorate accordingly. Let's say you opted for the ladybug and the ball is already a swirly red color. Use an outdoor paint to color in a head surrounding the finger holes. Add the outline of the body with at least one polka dot in the center. Add legs and antenna with more black paint.
Finish it off. Spray your finished product with polyurethane or another sealer to make sure it weathers the elements.