I don't work for Sainsburys but in my job it's very normal to work an 8 or 9 hour shift, sometimes more than that aha
Working an hour after the store closes means you'll be helping to clean up and prepare the store for the next day, sometimes you get let out early if there isn't much to do.
As for activities they'll probably get you to do some task in the store but it'll be simple and someone will probably help you, it's just to get an idea what it'll be like working in the store. If your interview is with other people then there might be some team building activities as well like work in a team to design a product and pitch it to the others.
Hope that helps, good luck with your interview