Best team building exercises for new recruits?

A swift kick in their collective asses.

Leader Reaction Course, that's the one with little obstacles they have to work together to solve. For maximum effect, you play joe. You can either rotate who is in charge, or let them do what they do and see who the infornal leaders are or who steps up to fill leadership gaps.

Challenge them to see who can stay off of their cell phones for the longest.