Obviously, a major consideration would be the mess factor. The experiment would have to be self-contained so as not to splatter everything in the cabin. Here's my idea:
Take a standard 16-oz bottle of Diet Coke (available after security!) and use a modified Spangler Geyser Tube with a clear plastic bag on the other end to catch the spray. The release mechanism would use a magnet to hold the mentos in place, then a quick shake should start things going.
This would look particularly cool if there's enough light for high-speed photography.