Are there any hidden items in the DVDs? Would be cool to find something like that.

There's an outtakes real on the bonus disc. I've yet to access it. I figured out how to unlock it on the PC, but haven't had the time to try it on the TV. The computer doesn't seem to want to play it.

The how and where are in an article Massassi has listed on the front page.

Outtakes:
Insert the Bonus Materials Disc. Enter the "Video Games And Still Galleries" menu. Press 1138. Note: For some remote controls and DVD players, you may have to press 11 (press +10 if required) then press Enter; press 3 then press Enter; then press 8 and press Enter. The box next to R2-D2 will be highlighted. Press Enter to view a series of outtakes from all three movies.

Alternately, insert the Bonus Materials Disc and begin viewing any of the featurettes. Then, advance to Title 38, Chapter 1.

Hidden trailer:
Insert the Bonus Materials Disc. Enter the "Trailers And TV spots" menu. Press 1138 on your remote. Note: For some remote controls and DVD players, you may have to press 11 (press +10 if required) then press Enter; press 3 then press Enter; then press 8 and press Enter. The "Trailer" option should change to "Revenge Trailer". Select that option to view the trailer from Return Of The Jedi when it was originally called Revenge Of The Jedi.

Alternate menus:
Insert Disc 1, 2, or 3. Normally, each movie will display a random menu when the disc is played. To change the menu manually, press Audio Change at the FBI logo to select the first menu, 2 for the second menu, or 12 (press +10 if required) for the third menu.

Is there an Episode III trailer on the disc?

If you really don't care which one you get, I found that just pressing the "next chapter" button while viewing those screens to get to the menu will switch the menu up.

Why would there be an Ep.III trailer? He hasn't even released a teaser yet. Has he?

I thought there was one. If I remember correctly they advertised one with the box set.