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. Just click on the lion's head on top, and that will bring up the Easter Egg. It's called Blonde Cam (it's about Skandar going around with a video camera talking about blonde people).