A couple of years back we bought a Sodastream, gas bottles and the sodastream syrups. They come in several different types from full sugar, low cal, and naturally sweetened. Very easy to pick and choose what suits you, and lots of flavours. Alternatively, you can just add a couple of crushed berries or a dash of normal sugar free squash to the bottle. My favourite was pink grapefruit, sweetened with stevia and sucralose (if I remember correctly). Incredibly refreshing, and caffeine free. Even just having fizzy water easily available is a pleasure. Ice and lemon, and good to go.
Actually, thanks for reminding me. I will dust the machine off as the weather warms up.