Mario Golf cheats and cheat codes for Nintendo 64

Left-handed character
Hold Z or L while selecting a character at the golfer selection screen.

Enable code entry screen (NTSC version):
Go to the main menu, highlight the "Clubhouse" selection, then 
hit Z + R + A. 

1st Camp Hyrule Cup
Go to the code entry screen and enter code: "0EQ561G2"

2nd Camp Hyrule Cup
Go to the code entry screen and enter code: "5VW689O6"

Nintendo Power Mario Cup: 
Go to the code entry screen and enter code: "KPXWN9N3"

Unlock 5 Bonus Mario Star courses:
Reach minimum  2200 course points

Change costumes
While selecting a character hold C-Up, C-Left, C-Down, or C-Right 
at the character selection screen.

Taunts
Hit C-Up, C-Left, C-Down, or C-Right during an opponent's turn.

Complements
Hit Up, Left, Down, or Right on the D-Pad during an opponent's turn.

Unlock Luigi: Beat Luigi in get character mode.
Unlock Yoshi: First unlock Luigi, then Beat Yoshi in get character mode.
Unlock Sonny: First unlock Yoshi, then Beat Sonny in get character mode.
Unlock Wario: First unlock Sonny, then Beat Wario in get character mode.
Unlock Harry: First unlock Wario, then Beat Harry in get character mode.
Unlock Mario: First unlock Harry, then Beat Mario in get character mode.
Unlock Bowser: First unlock Mario, then Beat Bowser in get character mode.
Unlock Donkey Kong: Find 30 stars in ring mode.
Unlock Maple: Find 50 birdie badges in tournament mode.
Unlock Metal Mario: Find all 108 birdie badges in tournament mode.
Unlock Gold Mario: first unlock Metal Mario, after highlight him at the 
character selection screen, hold C-Left and press A.