Probably something like tinned lychees, lemongrass, chilli and maybe a hint of mint leaves, all blitzed together then strained to get any course bits out, and topped with sparkling mineral water. I love refreshing drinks like this in summer. They're far superior to smoothies, which are better as a replacement lunch, in my opinion.
I like to keep a jug of various flavoured 'refreshers' in the fridge.
Basically, just mix and match a couple of fruits that you like, add some mint for that 'fresh' taste, and add anything else that takes your fancy. Top up with the cold, cold sparkling (or plain if you prefer) mineral water. Some good options would be mango and lemon; melon, cucumber & lime; orange, ginger & mint; orange, blueberry & mint; grapes, lime & mint.