First, beat the game. Then buy "Everybody Naked" at the bar. This secret costs secret tokens and can be turned on in the bonus options of the Extras section. If you enter the large gold doors on Nice Street and then go to the right of the doors you will find a man with a mohawk. The vibrantly-mohawked Nigel is located in several places throughout the game and he can sell you call kinds of unlockable items art, strip games, etc.
Leisure Suit Larry: Magna Cum Laude - Nude Mode (PC)
Unlock Nude Mode cheats for Leisure Suit Larry: Magna Cum Laude
Successfully complete the game, then buy the "Everybody Naked! The "Nude mode" option can now be toggled in the "Extras" section of the bonus options menu. Lose a game repeatedly until you get an option saying "Should we make it easier on you? In Larry's room, play "Penis Pong", which will give you 20 Confidence each time you win. It is also the easiest mini-game.