Things like melon or salad or various canned or bottled drinks. They also usually have a few things not on the conveyor like soup. Each plate usually has two pieces of sushi. Some types of sushi are more expensive than others. The plates are color coded so for example at the store near my apartment, Bikkuri Sushi which is a very large chain, they have 3 types of plates.
Some places advertise that all their plates are the same price.
Also, you don't have to take what is on the conveyor. You can ask one of the sushi chefs to make any sushi they have. These first 3 pictures are from Bikkuri sushi near my place. I used to go there at least once a week. Often enough that the chefs knew what I would order and just start making it as soon as I got in.