Stir together vinegar and sugar in a medium bowl until the sugar almost dissolves. Add watermelon, cucumber and cilantro; toss gently to combine. Just before serving, sprinkle with peanuts.

Tip: To toast nuts:

Heat a small dry skillet over medium-low heat. Add nuts and cook, stirring, until lightly browned and fragrant, 2 to 3 minutes. Transfer to a bowl to cool.