Regarding diet ..I am a vegetarian and have had no problems with food, but I also do take snack and soups with me just in case.
Clothes...I would say smart casual, I have done 4 JY Holidays but shorts and flip flop have not been the dress code, so would depend on where and what you are doing as in style of holiday booked.
I have done short and long haul flights for the holidays, and just take on board what you want to, I usually take a magazine (puzzle) and a book, but tend to watch the films and chat to the treaveller next to me..etc
I am sure you will work it all out, if not ring up and ask the JY reps advice.
Cindy