Update: I was unfortunately not aware of Shamus Young's severe criticism of Fallout 3 available here to link in the original piece and I regret that. It dovetails rather nicely with what I've written and it's much better executed than my piece. I strongly recommend anyone...
In order to get hidden levels, you have to go to the backpack, then to worlds, then you have compare the preview of the worlds with the worlds. You will find levels in the previews that are not in the worlds. (please note that I'm not talking about the levels that you have not completed) Then you go to the level that was on the path to the hidden level (in the preview) Finally, you find the crack(s) in that level. Go in it and find another crack, from that crack, you will go to the golden door.
In world one, there is a secret level. In order to get to it, you simply make a path from level 1-7 to a piece of tar diagonal from it.
In world two, there is also a secret level, to get to it, you make a path from the boss level to a pile of trash, located straight past the "Play!" sign.
Curiosity knocked the cat out
When you see a cat is chasing you, run away as fast as you can. When you see that the mouse is approaching a wall, this is your chance to take out the cat. Quickly dash so that the mouse is now running parallel to the oncoming wall, and the cat should smash into the wall and be knocked out for a few seconds.