I'm moving two sprites down along the y axis, each one the size of the screen, having some children images inside. Both sprites are flattened, and i perform per-pixel collision on each of its children (i don't do this every frame, but once every 100 milliseconds).
When one of the sprites reaches the end of the screen, i remove it from the display list and change its reference to another sprite of the same kind, but with different contents (these sprites are all pre-created and stored inside a vector). Then i position it at the top of the screen, to create a moving background effect.
I also have about 10-20 movieclips crossing the screen in the x axis, using the same logic of the sprites described above. I perform rectangle intersection collision detection with these mcs.
The collisions are always tested against the player, which has its position set every frame with a touch event (the image center is always at the mouse position).
Other than that i play sounds when the player collides with something and a background music is always being played.
This is basically what i'm doing.