A room decorated in a sports theme can make decorative yet practical use for tennis balls. Attached to a drawer, tennis balls can make whimsical pulls. Depending on the size of the furniture, you can choose to attach an entire tennis ball or cut them in half and get double duty from a single ball.
The same sporting theme makes a great home for a tennis ball cut in half that is then screwed into the wall to serve as a decorative doorstop. Not that you have to limit this unusual use for tennis balls to rooms with an athletic theme.
Placing a tennis ball into a basement drain is an effective way to keep creepy-crawlies from making themselves at home in your house. A tennis ball is heavy enough to obstruct entry of bugs from the pipes through the drain and into the house, yet is light enough to simply float upward when water enters the drain to be flushed out of the house.
Garage Early Warning Device
Suspend a tennis ball (or two or three) from the ceiling of your garage so that they hang down just low enough to bounce against the windshield of your car. This will serve to notify you when you have driven far enough into the garage to avoid running into any objects occupying the area beyond that space. This is an especially effective use of tennis balls when your garage lights are out.
Cutting a slit into a tennis ball can actually be a big help when it comes to removing stubborn screws or giving you some extra power when you need to screw something into place. Slide the slit over the handle of a screwdriver and grasp the tennis ball in your palm. Twist the ball to provide increased power to turning the screwdriver and this will give you additional might.
Why pay for a ridiculously overpriced massage device when you can use ridiculously cheap tennis balls to achieve the same result. Slide three tennis balls into a tube sock and tie a knot in the opening. Rolling the balls back and forth over your body achieves the same massaging effect as an expensive monotasking device sold in high end stores.
Inconspicuous Storage of Valuables
Create a tiny slit in a tennis ball that affords enough of an opening into which you can slide small things of value. This homemade safe can then be stored unobtrusively back inside the can you bought them in or seemingly tossed in casually with a bunch of other worthless junk and most thieves and burglars will be none the wiser about the cache of valuable property hidden inside.