Easter egg: On Disc 2, go to trailers, and select Theatrical Trailer #3. While the trailer is playing, input the following numbers in order (slowly): 5 6 5 3 7 (JOKER). The trailer will restart and show a modified version of the original trailer, with Joker scribbling all over it. This is a reference to the marketing campaign promoting the movie, as fans who completed the entire set of events were greeted with the third "The Dark Knight" trailer, modified by the Joker.
Easter egg: This Easter Egg is on the first page on the Extras DVD. Go between the Setup and More buttons, and a crown pops up. Click on it to bring up the Easter Egg. It's called Toastie (it's about toasted sandwiches).