What's the word for the sad-sweet feeling at the end of a great trip?

Dropped two old friends off at the airport tonight after their three-month adventure in America. We cracked bad jokes to break the tension, and they described their feelings beautifully: A sense of nostalgia for something that hasn't quite passed, a sadness, but a full kind of sadness, like a thing accomplished. That gets to the heart of what travel is, for me.