Bring water and food. The only thing I regretted was not having some food, at least I brought water. After the tour, my kids were starving and it didn't help that so many people around us were pulling out their lunch bags. There's no food to buy within the Vatican but you can bring your own. In retrospect we would have had lunch in the Square and then gone back in. It wasn't a cheap tour, but nothing in Rome is! For best bang for the buck, it was money well spent and served as an excellent introduction to the Vatican's treasures.