I brought a bag of Munchos in my carry on last trip down. They're packaged in a Mylar or Mylar-type bag. When I fished it out in flight, it had expanded big time with the pressure! It was hilarious! Like a big Munchos balloon! I was afraid it would make a big POP if I opened it, so I left it. I had visions of panicked fellow passengers covered in Munchos bits! (I'm a bit paranoid). I opened my carry on after landing, before we were docked at the jetway, and it has deflated totally back down. It was way cool!

That said, you could probably get more into your luggage. I'm with Capt D - the prices on the airlines are outrageous, and we always take something O/B with us for the flight - but I haven't tried to bring a portable bar yet ... great idea!