Unlock Island Jack:
In adventure mode go to danger island. when you get to the jungle go below the trees. when you see a giant snake wrapped around a tree branch near the end on the left side of the jungle shoot it and you will get island jack

unlock prof. gutenburg:
Go to adventure mode and go to torpedo run there will be three subs on yor radar one of them is a yellow sub with prof. gutenburg on the front shoot two torpedoes at it and you will unlock prof. gutenburg

Andre LaToilette:
In Bigfoot Mt., when you start in the beginning
of the level you will see a house on a hill. In
front of that house is a snowman, shoot the the snowman and if you shoot enough it will say you have unlocked Andre LaToilette.

Get Sheik Abdoul (the Arabian man with the rocket powered flying carpet):
In the Cave of Blunders, you start above a Arabian city; in the center of town there is a small area (with pillars) where Sheik is being bullied by a palace guard. Kill the guard to get Sheik Abdoul.

Cactus Rose:
In the beginning of Coyote Canyon on your right will be a town.In that town there will be a girl being hanged. Free her by shooting her and it will say you've unlocked Cactus Rose.

Unlock Sammy Wasabe:
Go to monster isle. Shoot a ninja by the shrine in the middle of the jungle. If you did it right it the guy will say "you unlocked sammy wasabe.

Usefull plane tricks.:
When you hold the triangle button and flick (or hold) the analog stick/directional pad up, down, left or right. (regular plane controls) You get...

triangle + up = "Dive Bomb" (pull up to stop the dive bomb when you desire)

triangle + down = "Somersault" (you use this in the mini game "Torpedo Run". You can't cancel out of the somersault when it's going. When you are accelerating you do a big one and a small one when not accelerating)

triangle + left = "Left Barrel Roll" (it will make your selected character do two rolls when you do this. When you hold the analog stick longer the left, you do a longer barrel roll. You'll automatically stop at a certain time when you can not barrel roll any more. It sort of works when you want to stop doing a dive bomb except that you continue going for one or two rolls then stop.)

triangle + right = "Right Barrel Roll" -see "Left Barrel Roll" description-

When you hold the L1 or R1 or both buttons and turn in the direction you want (left or right); you'll make sharper turns.

L1/R1 or both + triangle + down = "About Face" You turn around and fly in the other direction you were flying.

This is good to use if you accidentally go the wrong way. Holding L1+triangle+down makes a slow "About Face" You stay in that one spot and turn slowly (which wastes time) if not in Torpedo Run.

Holding R1 plus the other buttons make a fast "About Face" but it still takes longer.
Using L1/R1 plus the other buttons will make the best "About Face" you can do. So I recommend doing L1 & R1 + triangle + down combo.