Password Screen
At the Main Menu, select 'Clubhouse' and press A+Z+R. This will enable the password screen.
(Thanks to wildcats11@hotmail.com, ALMEHDA@aol.com)

Enable Donkey Kong
Get 30 coins in Ring Mode.
(Thanks to wildcats11@hotmail.com, ALMEHDA@aol.com)

Enable Bowser
Get all of the rings in Ring Mode.
(Thanks to wildcats11@hotmail.com)

Enable Mable
Get 50 coins in Tournament Mode.
(Thanks to GoFalcs@aol.com, wildcats11@hotmail.com, ALMEHDA@aol.com)

Enable Metal Mario
Get all 108 Birdie Badges.
(Thanks to wildcats11@hotmail.com, ALMEHDA@aol.com)

Left-Handed Golfer
Hold L when you choose your golfer.
(Thanks to wildcats11@hotmail.com, ALMEHDA@aol.com)

Extra Yardage
Keep tapping Z or A when the ball lands to pick up a few more yards.
(Thanks to wildcats11@hotmail.com)

Different Outfits
Hold any of the C-Buttons while choosing a golfer to get a different costume.
(Thanks to wildcats11@hotmail.com, ALMEHDA@aol.com)

Taunt
Press C-Up, C-Left, C-Down, or C-Right to taunt your opponent during his/her turn.
(Thanks to wildcats11@hotmail.com)

Cheer
Press any direction on the D-Pad to cheer for your opponent during his/her turn.
(Thanks to wildcats11@hotmail.com)