sports! gymnastics, breakdance, acrobatics...
One week without any sports? no way! Train trip is a great possibility to learn new things. Standing on hands, breakdance moves, standing on other person's shoulders... Anything that can be done in a small place as a train car.
Enna Rane commented
I could maybe teach some pair salsa :) Salsa is often danced in small hot clubs, so why not in a train ^^ (But I need partner to set an example for the class. Any volunteers? :)