there may already be a thread for this but I don't feel like sorting through hundreds of pages to find it. Anyway I discovered awhile back that if you repeatedly tap the use item button when you hit an item box the item will appear faster. this works on both the DS and the Wii. I haven't tried it on any other versions. That's the only one I know so if you have anything to ad feel free to post.
In Mario Kart Wii, if 2 players want to play as the same character, have one select it and deselect it, and have the other player do the same. Then, go back a window, and back to where you were. If you did it right, both players should have it selected.
There's one on wii maple tree way from the start u go right and there's a secret path first it slows u down so u get a good power then it boosts u up so ur like in fifth with a good power but it's rare to get a bullet loads of other but I'm to tired to explain so ask me for more on my comment box on my page
In Grumble Volcano,when starting a time trial,drive up to the starting line.Then aim yourself to the left.There should be a little bump.Use a mushroom when aimed to it,wheelie(because this can only be performed on a bike),and you might get on top of the little chunk of rock by the start.Then drive up past the first line(it's visible,don't worry),then turn 180º and drive forward,past the halfway point,and past the line at the second end.Then drive forward on the opposite side,and after you past the first line,the game will count it as a lap.This requires practice,so don't break the controller after 5 minutes.If you get good at it,you can finish Grumble Volcano in under 30 seconds.