Too many bugs
I like what you've done here, trying to combine many simple games into one file, but there are too many bugs in it. To name a few:
The DK timer does not reset to zero when you start a new game.
DK can move through the ceiling.
DK can jump in mid-air (I know the actual game allows this, but only with a roll, which is something you have not included).
In the diving game, pressing space bar to retry makes the instructions flash up very quickly - so you can't re-read them if you want, OR make them disappear if you want.
In the racing game, you can travel THROUGH opponents' cars.
The swimming ans comet games are much too easy.
In the tennis ball game, the "restart" thing flashes wherever the mouse it (which probably means off the screen) but you don't get the chance to click it - so what's the point? At least make it flash in the middle.
In the bouncy game, a high bounciness difficulty means you're almost guaranteed to fly off the screen.
Also, in the bouncy game, the yellow player starts off with 4 points and the red gets 2 -- not fair.
The multiplayer games button breaks it!
That makes it sound a lot worse than it is, but hey - someone has to find the bugs! :P