Take things like a DS system or iPod touch so you can play some games and listen to music, some embroidery floss to make friendship bracelets(even if you just end up giving or throwing them away later... it_s cheap entertainment and will keep you busy a few hours), maybe a few magazines. And I_d also suggest arranging your schedule before the long ride so that you are tired and can sleep a good amount of the trip.
Heck, talking to each other would be fine too, just as long as you two have something to talk about. I suggest for you two to bring a set of cards to. There_s plenty of card games you two can play while on the bus. Maybe bring chess, checkers, etc. Although sitting side by side may be a bit hard, but you can always turn your bodies a bit to face each other.
If you two have phones, text other friends or call to converse with them too. Maybe bring some sort of game system so you can pass time.
music and watch movie tv shows and music videos. Hope this helps have fun on the trip.