Here are 3 of the hidden things (so-called easter eggs) on the Seinfeld DVD set that I have found/been told about so far. They're all from the Seasons 1&2 set.
1. An early camera test - On Disk 1, from the main menu, select "extras". Then press up twice to highlight the jalopeño and select it. This is just a camera test with Jerry walking around the apartment so that they can adjust the lighting and colors and such.
2. "The Bet" - On Disk 3, from the main menu, select "extras", then select "inside looks". Hit left once to highlight the fortune cookie and then select it. This is the story of the never-aired episode "The Bet." It dealt with Elaine getting a gun and they scrapped it fairly early in the filming.
3. Split-screen - On Disk 4, from the main menu, select "setup", then scroll down to episodes and hit up to highlight the green pea. Select it. This takes you to a split-screen demonstration comparing the original scenes to the digitally remastered DVD version.
"Dear Joan, I wanted to say that I'm sorry for the screamin' last night and the nights before. I've wanted more from this, than anything I've ever known..."