Cut watermelon into 1-inch cubes, removing any seeds if necessary. For the cucumbers, either peel them completely or create a striped effect by partially peeling lengthwise. Halve the cucumbers lengthwise, scoop out seeds, and cut into cubes roughly the same size as the watermelon.
Thinly slice the red onion and roughly chop the mint leaves just before adding to preserve their aromatic oils.
In a small bowl, whisk together the olive oil, fresh lime juice, honey, salt, and pepper until emulsified.
In a large bowl, gently toss the watermelon, cucumber, red onion, and mint with the prepared vinaigrette. Use folding motions rather than stirring to keep the watermelon from breaking down.
Sprinkle the crumbled feta cheese over the salad just before serving. For an elegant presentation, add the feta to individual servings rather than mixing through the entire salad.
If using, drizzle with balsamic glaze right before serving for an extra flavor dimension.
For best results, chill the salad for about 30 minutes before serving to allow flavors to meld (though this step is optional).