Enchantable water bottles

Water bottles (and potions, if you hate having potions) can be enchanted to produce 2 exp bottles per level spent.

On average this gives 14 experience points per level spent. Levels up to 16 require 17 points per level, and levels after that start requiring more, so it will generally be a net loss. I'm thinking of adjusting the quantities so that you get on average as much experience as you put in.