Also of course, Cider will dehydrate you so thats a bad choice. I'm knocking back some lovely ice cold (and I mean ice cold) blackcurrant drink I mixed earlier and popped in the fridge (always in this heat have at least 3 litres of it in there).
I also change it around for orange drink sometimes to keep the flavor fresh.
If I go to an airshow, I do the same thing but pop them in the freezer (allowing some space in the bottle for expansion) and they are delicious frozen, always reminds me of the orange Jubbly we used to buy from the sweet shop (frozen of course), fantastic on a hot day.