When I did my M&G a few years ago, I had the band sign an 8" drum head (made sure it was the same brand MP used) and that worked out great. Easy to carry around, good room for all of them to sign, and MP even drew up the DT logo in the center. Of course, I'm a drummer so it made the choice of a drumhead easy.
IF they get back to Phoenix on the (hopefully) 2nd US leg and I am fortunate enough to do the M&G, I'd have them all sign the ADTOE litho from the box set.
As for questions, well, I'm originally from Long Island near where JP is from, so I was able to talk to him about that. I talked to MP about TSCO and what it meant to me because it was the 1 year anniversary of her passing (MP said he'd dedicate the song that night to my mom, which meant a lot to me when he did that), and I very briefly talked to JR about his playing style. Didn't have much to say/ask JLB because while he was signing I was still talking to JP, and I just said Hi to JMX cause I figured that's all I'd get back anyway.
Definitely a good idea to come up with some questions beforehand, but realize you MAY only get one question with each member (if that) because they move you through the line pretty quick. If you have more than one, try to ask it while they're setting you up for the band photo.
Oh, and HAVE FUN!