Watch movies, talk about the movies, sing One Million Bottles of (Something) on the Wall to drive the bus driver crazy, listen to great music, discuss the great music, and if the bus ride isn't too bouncy, might I suggest painting your toenails? Just enjoy everybody's company and have a great time. Bringing a joke book, and reading them out loud, and then encouraging everyone else to tell a funny/lame joke, is a great way to generate laughter:)
Bring handheld video games, iPods, your cell phones, tablet, etc if you can! Watch videos and play games together on them :) You could also bring a notebook and do something where one person starts out writing a sentence, and then you pass the book around and everyone adds on until you have a story. You can play tic tac toe with the notebook, have a drawing contest, etc. You could bring a deck of playing cards or Uno cards to play with, or magazines to look through, and just talk!