I have a suggestion, that your bus collects litter inside during the course of its service. As passengers leave the bus, newspapers, cans, etc are left on the floor and on seats. Passengers getting on the bus may complain if the inside is dirty.
Pieces of litter could be removed by clicking on them - giving the requirement for the driver to 'walk-through' the bus at termini and inspect the seats.
I had the idea today when I was sitting at a taxi rank and saw a bus driver eject litter from his bus by hitting the brakes when he was going round a roundabout with the doors open. I notice the drivers' cab door swings around as you drive if it is not kept shut - does this mean that it is possible to have 'loose' litter items - such as bottles rolling about the bus?