Insert the first disc of the DVD set in your player and from the Main Menu go to the 'Special Features' section. Now highlight the menu entry 'Main Menu' and then press the 'Right' arrow key on your remote control. This will highlight Superman's 'S' on his chest. Now press the 'Enter' key and you will get to see a hidden bonus commentary for the episode 'Apokolips... Now! Part 1' featuring Bruce Timm, Glen Murakami, Dan Riba, James Tucker, and Paul Dini.
Insert the second side of the second disc of the DVD set and go to the 'Special Features' section. There enter the 'Trailers' section and select the Superman 'Hero' trailer. Play this trailer to the end and it will be followed by a teaser trailer for the new 'Superman Returns' movie!
All easter eggs were generously submitted by SirDonk