MIA: Well, I must say that was quite exciting.
LORELAI: A little disturbing. I think the whole town needs a field trip.
A field trip is an excursion by a group of people away from their normal environment, most usually used in the context of education (what might be called a school trip or school tour in other countries).
Lorelai seems to be saying the whole town needs to get out of town, so they can learn how other people live. It’s the closest she gets to saying they should try to see things from Jess’ point of view, and she does seem to be bothered by how the town reacted to him.