While in Venice, after talking to Elsa and you are in the library, exit and go back to where you first arrived on the piazza. Go as far left as possible and you will see a "romantic couple" drinking wine. Try to pick up the wine bottle and you will see that you cannot. After that, "Look" at "Wine Bottle" and Indy will say something about the wine similar to "1924...not a good year". The man will say "I knew that!" Then "Pick Up" "Wine Bottle" and you will have it.
Don't worry if you fail to get the tickets from the man in the airport. Go outside and study the biplane manual. Get in, turn it on, then start flying. You do not need to get on the Zeppelin because you will end up getting off and using the biplane eventually.