There is also an Easter egg on the second disc of the Fellowship of the Ring extended edition and the second disc of the Return of the King extended edition.
On the second DVD, scene 48, click on the number 48 - on the right side of the screen - and the silhouettes of Barad Dur and Orthanc appear just underneath, click it. You'll see another introduction by Peter Jackson, and the Two Towers cinematic preview trailer: the one played in theatres after the Fellowship of the Ring, towards the end of its running period.
For Return of the King disc 2 easter egg, scroll to the end of the chapter selection page and press down once more on the bottom one to highlight a hidden ring icon; select the the ring icon and the Easter egg will begin to play. (the MTV movie awards spoof interview with Peter Jackson, Ben Stiller and Vince Vaughan.)
Does any one know how to find the Two Towers Disc 2 Easter egg?