Mine the coupons for a promotion by interacting with adverts. Coupons are mined on exponential basis so need more and more people to interact before unlock.
Use the touch capability to whack-a-mole with a company logo or something. High scores shared between all adverts.
Collect the adverts, fight the adverts, top trumps style
Advertise the fact that you want to make friends or just talk to someone on your journey. Add the bus number you're taking and match the people with realtime chat.
Like foursquare, but with clearchannel adverts.
Rate the advert, influence what's shown. Maybe it takes it out of rotation for a bit, maybe it just changes what the advert displays.
upload your notice to the noticeboard, sponsored by [company], maybe these need to be moderated...