Using a sharp knife, cut off the top and bottom of the watermelon to reveal the red flesh and create flat surfaces on both sides. Stand the watermelon upright, and slice off the remaining peel so only the red core remains.
Cut the watermelon into roughly 1-inch (2-cm) cubes and transfer to a large bowl.
In a medium pot over high heat, bring the balsamic vinegar to a boil and reduce until roughly ¼ cup (60 ml) remains and the vinegar has thickened significantly, about 10 minutes. Remove the pot from the heat and set aside to cool.
Arrange the watermelon cubes on a plate or place in a large bowl.
Arrange the red onion, mint leaves, drops of balsamic reduction, basil, and thyme decoratively over the watermelon. Or, if using a bowl, toss the watermelon, onion, and herbs together, then drizzle with balsamic reduction.