New Balls
Those weapons you call clubs won't do much good without ammo.
Shag Balls
Rain Gear
This includes jacket, pants, hat and rain gloves. Make sure you have a cover/hood for your bag so your sticks stay dry, too.
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Umbrella
If you're an avid golfer, you're going to get caught in a storm on the course at some point. Be prepared when the sky starts crying.
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Business Cards
First-Aid Kit
Sunscreen
A startling amount of golfers battle skin cancer. Do your part in prevention.
Markers
Not only to label your ball, but can be a de facto pencil for the scorecard.
Towels
Scorecard
Tees
Extra Socks
Maybe you're playing 36 and want to change between rounds, or you step in a puddle or lake and don't want soggy feet the rest of the day. Just make sure to take out the used pair after the round, or else risk a smell stewing in your bag that's worse than death.
21 of the best golf socks you can buy to upgrade your sock collection
Cash
Coins
Gloves
Three is a good number: One for practice, one for play and one in reserve.
Energy Bars
Water Bottle
Make sure it's always full before heading out, in the off-chance the water coolers aren't filled on the course.
Editors' Choice 2020: Best Hydration For Golfers
Related: The best self-sanitizing water bottles that are perfect for golf