1) Cut an X in the top of each ball and put them on the bottoms of chair legs to cut down on noise and floor scuffs. If you don't have scuffable floors, many schools take donations for just this purpose.
2) Donate them to a local nursing home for use on residents' walkers. They make the walkers easier to push around for people who aren't strong enough to lift them.
3) Hang one on string from the garage roof to help you park without running into things. When it touches the windshield or rear window, you know it's time to stop.
4) Keep certain types of gnats or flies away from you when you are outdoors. Just cover a tennis ball in Vaseline and hang it from a tree or bush.
5) Tennis balls with holes drilled in them have been used in the UK as protective homes for field mice.
6) When packing something for shipping in a box that's too large, use tennis balls as shock-absorbing cushions that will hold the item steady in the box.
7) Use them to remove scuffs on floors. Many janitors use this trick by placing a tennis ball on the end of a broom.
8 ) Protect your surfboard when it goes on an airplane journey.
9) Play a creative catch game that will amuse kids to no end.
10) Throw a few tennis balls into the dryer when you are drying comforters, fluffy coats, pillows, or anything else that could use a good fluffing.
11) Tennis balls can also help any laundry load dry faster; throw two or three in the dryer and your clothes will be done quicker.
12) Speaking of laundry, put a tennis ball into your washing machine along with your shower curtain and 1/2 cup of vinegar, then wash with hot water. The vinegar will kill the mildew and the tennis ball will help to scrub the mildew off.
13) If you or your partner snore, attach a pocket to the back of the snorer's pajamas and secure a tennis ball inside. This will ensure that the snorer sleeps on his/her side -- most people snore only when sleeping on their backs.
14) Make your own juggling clubs.
15) Make a very cute pencil/mail/phone holder.
16) Cut a slit in one and use it to cover the trailer hitch on your truck.
17) Hide stuff in them. Make a slit in a tennis ball, then squeeze either side of the slit to open it up. Place money or other objects inside, and release to close the opening.
18 ) Use the same concept as above to pass notes or other items over long distances. This idea has been used at auctions to pass receipts to winning bidders.
19) Cut a portion of the ball off so that it will fit over the sharp corner of your coffee table. Repeat for the other corners to baby-proof a room.
20) Make a beautiful flower for your sweetheart. You can even fill it with candy.
21) Slit a tennis ball open, insert some beans or jingle bells, and seal closed with glue or rubber cement. Give it to a toddler as a musical instrument.
22) When you are seated, put a tennis ball under each foot and roll your feet around on them. They make wonderful massagers.
23) Put two tennis balls into a large sock. Tie the sock securely, then use the contraption as a back massager. This is a great tool to have in your hospital bag when you have a baby, since concentrated back pressure can help to relieve a great deal of labor pain.
24) Make bizarre furniture.
25) Use tennis balls to anchor clusters of helium balloons at parties. Knot together a group of ribbons attached to balloons. Cut a small X in the top of a ball and insert the knot. Fill the ball with sand if you want extra security.
26) Learning to juggle tennis balls can improve your hand-eye coordination and visual reaction time. It can also help to keep your brain sharp.
27) Put tennis balls on the tops of poles to mark the edges of your driveway or drainage ditch. The bright yellow balls will be visible in the dark and help you avoid driving into the ditch or over the grass in the dark.
28 ) Put a tennis ball on the end of a broomstick and use it to clean cobwebs from the ceiling.
29) Wrap a piece of sandpaper around a tennis ball. It's easy on your hands and can be used to sand curves on furniture or woodworking projects.
30) Make a unique ornament for your home or to give as a gift.
31) Make an incognito squirt gun.
32) Prevent your bike's kick stand from sinking into soft dirt by cutting a small slit in a tennis ball and sliding it over the kick stand.
33) If you find that the legs of your lawn chairs get stuck between the slats of your deck, put tennis balls on the bottoms to keep them where you want them.
34) Keep the yuckiness out of your pool by floating some tennis balls in the water. Supposedly, the balls will absorb body oils from people who swim in the water. You need to replace them every few weeks to keep them fresh.
35) Cut a tennis ball in half and use it to get a better grip when opening jars. Just place the ball half over the lid, and the rubber on the inside grips the lid to help you rotate it easier.
36) You can apply the same concept to screwdrivers to give you a better grip. Simply cut a slit in the tennis ball and slide it over the screwdriver handle to give you a better grip.
37) Make a tiny stereo.
38 ) Ham radio enthusiasts with gigantic antennae on their cars can use a strategically placed tennis ball to keep the antennae from ruining the paint on the cars.
39) To keep a door knob from smashing into and damaging an interior wall, cut a large slit in a tennis ball and slide it over the knob. This trick also works great to keep curious toddlers out of off-limits rooms ... until they figure out how to squeeze as they turn.
40) Use a tennis ball to explain internet security (see video in source link below).
41) Squeeze a tennis ball in your hand whenever you have an extra few minutes to increase your hand strength.
42) If you want to leave your car door open but don't want the interior lights to run down the battery, just wedge a tennis ball into the door frame to keep the light switch depressed.
43) When fueling up your car, use a tennis ball to keep the handle of the gas nozzle pushed in to avoid painful hand cramping.
44) Make a snowman ornament for your holiday tree.
45) Explain and illustrate molecular structure.
46) Make an awesome pocket tripod for your small camera.
47) Build a model trebuchet and hurl tennis balls into your annoying neighbor's yard.
48) Make talking apple puppets. These will amuse kids to no end.
49) Put tennis balls under the windshield wipers of vehicles that will be stored for long periods. This will help the blades last longer.
50) Use the time-tested method for finding your car in a crowded parking lot: put a tennis ball on the end of the antenna.
A words of caution: tennis balls should not be used as dog toys. The felt that covers them can wear down a dog's teeth. Larger dogs can choke on tennis balls. Ask your veterinarian for advice on alternative toys.