Nothing. Fruit makes its own juice, especially if you include a very juicy item like melon. Sometimes I will get a passionfruit, and scoop out the inside over the fruit. (Tip - a ripe passionfruit should look wrinkly and horrible. A smooth passionfruit is not ripe.)
Personally I don't like the idea of adulterating fresh fruit with artificially sweetened things. But that's just me - I am sure other people will have lots of ideas for you!