Bring them lunch. Or even a soda. Or even just a glass of water. Most recruiters have very little time to get away from their booth. There is a line a mile long and it is not getting any shorter. If you notice that they are in need of something, you can either ask them if they want you to get it or just get it for them. They will be eternally grateful, and it may be what sets you apart from the crowd at the end of the day.