One I can think of on to of my head would be Silver Tea, which is water + sugar/honey + milk. It's a rather simple one, but it tastes good. Next would be hot milk and chocolate milk, All of these would use already existing items.

hm, never heard of silver tea before. can you even call it a tea if it takes no tea leaves nor herbs? :P
I won't add hot chocolate because Simply Tea already has a nice one, so it's a bonus reason to use both mods together!
hot milk is interesting, I like the idea of a simpler milk drink. thanks for the suggestions, and sorry for the delay!

Silver tea (silverte) is, as far as I can see, a Swedish thing, and is great for a sore throat, and coughs and the like. It's boiling water, milk, and some type of sugar. If done correctly, it tastes really good. Edit: It uses cold milk, btw.