You should always have a day pack on Kilimanjaro. With your day pack, you can carry extra clothing, food, water, first aid kit and any other bare necessities. If your day pack is not waterproof, or you don’t have a rain shell for it, then you can take a plastic garbage bag and line the inside of the pack. This way, if it rains (and it will rain), you are protecting everything from gettting soaked. I think, even if you do have a water proof pack, you should still create a liner, like the example in the picture, with a big plastic bag. It’s just a way to ensure that everything stays somewhat dry. To be extra cautious, you might also consider a day pack weather cover.