First to clarrify, there are NO cheats in this game (unless you use turbo when racing the boulders). But here I am where I will give you awesome tips and unlockables!
You get a lot of stuff after beating the game, so here's a complete list of them with descriptions!
Complete the game once to unlock an all-new menu with an awesome interactive backround. To interact with it, move the thumbstick left or right to controll the scrolling of the backround, also hold the R button to zoom in and the L button to zoom out. Let's not forget that when starting a new game you can choose "Proffesional" mode. New mini-games are also there which are "Assignment Ada", and "The Mercenairies".
Play an all new game at a VERY HARD difficulty setting. The scenario and item placements are the same, but enemies are harder to kill and (and more powerful), and racing boulders is tougher this time around.
After beating the game, record a clear game save to a memory card inserted in slot-A of the GameCube. Then load this save to play another game, with ALL of the items, treasures AND pesetas that were with you when you finished the previous play through(kinda makes you wish you didn't sell some of you weapons in the last game before the last boss fight, huh?)! Before you start the game again though, the merchant's menu opens up, and you can buy or sell stuff before playing. New bonus weapons (I'll cover them later in here) are added to the shopping list so get that awesome gun and get started!
Only two minutes remain untill the chopper arrives. To delay the chopper, you must pick up hourglasses to increase the time remaining. The longer you stay in the game, the more likely you're higher to score (just don't die!) and get a better ranking. Score 30,000 points or more to recieve a four-star ranking AND unlock a secret character! Score 60,000 points or more though, and get a perfect five-star ranking!
To unlock this gun, clear the Mercenaries mini-game with all five characters, scoring 60,000 points or more to achieve an S Rank with each.
2.If you shoot out the lanterns in the underground passage, the place will go completely dark and you'll be able to see jewels on the ceiling.(Chapter 1-3)
3.From the hole in the ceiling you can go to where the Weapons Merchant is.(Chapter 2-1)
4.If you hit Luis five times there is an event that will occur in which the two of you have a falling out or a quarrel.(Chapter 2-2)
5.If you use the flash grenade on all "abnormal living beings" (all types), then you can defeat them in one hit.(From Chapter 2-1 on)
6.If you defeat the two lookouts in front, then the sling-wielding force won't appear.(Chapter 4-1)
7.You can destroy the creepy crawly's nest (a ton of items will fall out).(Chapter 4-1)
8.In front of the skeleton in the room with the Key Items it is possible to acquire a total of 3,000 pesetas.(Chapter 4-3)
9.If you examine Saddler's chair you will be able to pick up an emerald. If you check it again after that, Leon will sit down!(Chapter 5-3)
10.At the first jet ski jump, if you push the L+R button and jump just before you do this, you will rotate or spin around once. Even in jumps after that, if you push L+R buttons and jump, then you'll rotate off to the side. (Final Area)