Instead of adding unusable bowling balls to the landfill, display them around your home.
Fill the finger holes with wood putty and allow to dry thoroughly. Meanwhile you can decoupage the half of the ball that doesn’t contain the finger holes. When you have half the ball decoupaged set it in a bowl to keep it from rolling around and allow to dry. The next day you can decoupage the other half. When the glue dries apply several coats of clear shellac, varnish or polyurethane allowing to dry thoroughly in between each coat.
Place a bowling ball in an outdoor garden, arranging so that the holes are facing down into the ground or into a plant pot.
You can choose to fill the finger holes with wood putty before painting (or not). Apply silver, gold or copper leaf to the ball. Place in a basket or bowl to keep it from rolling or arrange one or more in your garden.
Fill the finger holes with wood putty and allow to dry thoroughly. Use a strong adhesive to attach small stones, glass tiles, broken dishes and/or other mosaic materials to the ball. When half the ball is done set it aside in a bowl to keep it from rolling around. When the glue dries finish applying mosaic to the ball. When glue is dry apply grout.