Fresh fruit is good for you no matter what. Still, some common high-sugar foods in fruit salad -- calling you out, grapes -- can be reduced and/or replaced by kickass berries (seriously, pick any kind) for powerful flavor even as melon offers a nice, mild balance. Watermelon, which has plenty of sugar, has a low glycemic load.
Truth is, you shouldn’t worry much about this one no matter what. Make a fruit salad. Then throw some fresh herbs in there. Ooh, you’re getting fancy. How about an acid like lemon juice or red wine vinegar? Maybe some finely diced pickled watermelon rind for crunch? Or even healthy fats, like feta cheese or diced avocado? Now that’s the right way to make fruit salad complicated.