One especially pleasing extra is marked "Improbability Drive". The first time I hit it, the screen scrambled, turned into a rubber duck, and then gave me a sing along for "So long and thanks for all the fish."
I went thru all the extras. The extras include deleted scenes and "Really deleted scenes" which are fake deleted scenes kept for the DVD.
I also listened to the commentary for one of the scenes. The TV Marvin that appears in one scene is not a reproduction. They had in fact found the original Marvin from the TV series in a box in a BBC warehouse in pieces. It took them a few days to reassemble him, but it is in fact the real and original Marvin from the old series. (Shiny!)
Then after discovering the Sing along is an extra... I realized that the improbability drive takes you randomly to one of the extras.
Love it. Will watch the film again soon :)