Do what I did. I told them that the elevators in America cost about 25 cents to use, every elevator is different so be sure to just carry a lot of change.

omg, I had so many kids run on facebook pissed as hell at me, yelling at me that they actually brought change onto the elevator looking for where to put it. hahahahaha. oh I'm cruel.

But on a more serious note there really isnt anything you can teach them or help them prepare. They will learn from experience (which is why they are going). It would take an hour to teach one concept such as how to take a bus, where as they will learn within seconds of actually being there.

draw them out a map of places to see and foods to try...and maybe of places to avoid. Just know that these kids are going through real training courses sponsored by real  qualified educators, and have their parents helping them prepare.

There was a really awesome ppt dedicated to NEW YORK CITY on this site.  It really went into great detail about the city.  maybe you can find it.  i'll do some looking here in a sec.  if i find it i'll post the link.

LOL to the elevator story!
I had a group go abroad. I spent time with them on various subjects to help them prepare for emergencies (What is the emergency number? How do you ask for directions?) or if you get lost/ simply confused.
We discussed cultural differences in relation to manners. We did everyday politeness and table manners, seperately. We talked about things that shocked me in Korea and what might shock them abroad. We talked about questions people might ask them that they might find offensive ("Do you eat dog?") and what to do about it (One kid said "Punch them in the face!" Another said, "Yes, don't you?") I suggested they choose English names and made a point of misspronouncing their names so they were prepared. Half opted for English names, half didn't, that was cool. The fun thing about these lessons was that the way the interact with me changed. They try to say my name correctly and they try to ask me creative questions about my home instead of "Can you eat this?" Anyway, it was cool to watch them grow.
I also showed them videos about tourist destinations and used them as a listening exercise, since they will need to brush up on listening. We talked about accents, since mine is different and listened to examples. Had an accent-mimic-ing contest.
But my favorite thing was that the first day I had them write a letter to me listing things they were excited about seeing and why they wanted to travel abroad. Then right before they left we talked about those two questions and looked over the letters to see how their expectations had changed.
I hope these ideas help