I don't mind plain water too much. I have that with plenty of lemon and lime slices in.
I love the Robinsons no added sugar cordials: fruit and barley tropical flavour is on my desk at the moment, but any will do. I have a couple of pints of that a day at work. I also have one or two cans of diet coke (usually mid-morning and mid-afternoon) and in the evening I'll often have slimline tonic water minus the gin. Often have sparkling water too, when I remember to buy it, that is. Sounds like a lot but actually I feel like I should be drinking more!