If I gave you all my unreleased cheats, then I wouldn't have the advantage in internet matches where someone was harassing me. However, I will give you this one.
For my favorite one, when I hold minus, then when a car gets hit with a bomb it goes through the floor and keeps falling for the rest of the race. That's the only one that was truly my own creation. My other codes are just mods of other people's codes. (Like the free float one is a mod of AdMan's Moon Jump code.) I'm not going to release my special bomb code until online Mario Kart Wii is totally dead.
It would be exceedingly difficult to create a cheat code to modify a character model. However, it is very easy to ROM-hack character models to make ANY custom character. (Yes, a VinsClone too!) All that is needed is basic 3D modelling skills, BrawlBox, Wii Scrubber, and Dolphin. You can see a list of already-made custom characters here: http://wiki.tockdom.com/wiki/List_of_Custom_Characters/Vehicles