Well you can look out of the window at the passing scenery, recommended for the first trips. Learning the route can be very important if you later choose a different time filler. You learn not just the landmarks, the stops and who joins where but how the bus moves, the way it sways and rolls how it breaks and pulls away. You learn the texture of the road, where it is smooth or rough, the particular sharp jiggle generated by repaired pot holes. Dips and banks so that you move with not against the motion of your ride. With time you can almost feel part of the bus rather than just a passenger.
Reading on a moving vehicle can be difficult and nauseating, it is a good way to incur travel sickness however, it is a great way of making use of your time being consumed by travel. I have not previously been able to do this but I tried a few moments at a time and once used to the motion of the journey a few moments more until it stopped bothering me. Who knew reading on a bus was a skill you could develop. Oh and used bus tickets make good bookmarks, told you I found a use for them.
You can have those conversations with bus friends of acquaintances that I mentioned earlier or you can take a nap. Bus napping is another skill which takes some development if you are to arrive at your destination and not continue past your intended stop.