As a youngster, I used to use Lift instant lemon tea - nice cold and very sweet. Nowadays I use water or a weak solution of fruity barley water. From what I've read, home made mixes with sweet drink and salt are more effective than the stuff sold in shops - partly due to the branded stuff making the drinks less salty to keep the taste more palatable. If I think I need salt, I munch on a salty snack like crisps or salted nuts.
I saw a programme on the TV some time ago (Dr Alice Roberts was in it) where they dehydrated people on exercise bikes (weighing them to see how much by) and then rehydrated them with either water, propriety isotonic drinks or milk and then re-weighing them - I can't recall it in detail but the upshot was that milk rehydrated better than water or the isotonic drinks (which performed as well as each other). The theory behind this that was offered was that because of the protein emulsion in milk, the body spent longer to digest it which in turn meant that the body had longer to absorb the water content (rather than it going 'straight through').