What to do with five litres of milk

Because our family is quite shrunk, and because two of us were too sick to be drinking much milk for a couple of days, we now have about five litres of milk that will go sour tomorrow.

So far I've made a litre of custard with it, but I'm going to have to think of more to do with it if I want to use it all up. Hmm: baking, milkshakes, puddings. I think there might be enough to do to use it all up, if I don't get tired of cooking!