If you ever get tired of the old colors of the characters to choose, when choosing your character click and hold UP for the colors to change. The names and abilities of each character remain the same though.
If you want to dive deeper than usual, when selecting your character and jet ski options go to custom and put the "Top End" and "Engine" bars all of the way. When you jump off a ramp or wave press and hold up and you will do a deeper and longer dive than normally.
When trying to do a double back-flip, you will realize that it is nearly impossible. But, if you choose Boog as your character, he can perform tricks extremely well. Go up off a ramp and do a normal back-flip, but hold DOWN on the analog stick keep on rotating. If you keep on practicing, you can perform a double back-flip. The bad thing is you can only perform it with Boog.
The Helicopter spin, being sideways while doing a backflip, may seem almost impossible to pull-off. There is a way to do it though. Its easiest with Boog, since he can maneuver the best. First go towards a ramp, but at an angle where you will hit just the edge of it. If you hold the control stick left (if going off of the left edge), or right (if going off of the right edge), you will notice you are now sideways in the air. Then you must do what you would do to perform a normal backflip, press up, then press and hold down. You will successfully complete the Helicopter Spin which will earn a lot of points.
Normally the only way you can do a trick in the air is by going off a ramp. There is another way to do tricks like the ones you can do off of a ramp though. Waves. Waves are found on most courses on the hard difficulty setting. You can treat these waves like ramps though because they lift you off into the air briefly, and depending on how you hit the wave, it is almost just like going off of a ramp!
Click and hold down both gas buttons (A and Z) just before the announcer says "GO!". If performed correctly you will start out with maximum power and a little boost.
If you are in a race, and don't need to pull of tricks or just want to get past the annoying waves, press and hold B right when you are about to hit them.
If you finish championship mode on Expert and finish in first place, you will have unlocked reverse mode. Whenever you are playing championship mode you can select reverse mode to play the courses backwards.
To unlock Twilight City, clear the Hard mode.
Select Championship under the Expert difficulty. If you get to the seventh stage (Glacier Coast), and finish it, you will unlock it for Time Trial, 2P VS., and Stunt Modes.