Student hosts are great! Most live within minutes from the med school, and all of mine have walked me to the interview room in the morning. Staying with a student has helped me learn so much about the school, and I feel I've been more relaxed as a result. About the airport question, I agree with the previous posters. You can ask, but if the city has good public transportation (ie..NY, Boston) then asking would be kind of rude. You should just ask for directions from the airport, and if they wouldn't mind picking you up, then they will offer.
Don't forget the thank you gift! I think a gift is polite and it's really the least you could do. Or take them out to dinner...just make sure you thank them.