Combine the balsamic vinegar and sugar in a saucepan over medium heat. Bring to a simmer, stirrring occasionally until the sugar is dissolved. Remove from heat and set aside to cool. Arrange the skewers starting with the watermelon squares, then a basil leaf and finally a tomato half. Repeat. Arrange skewers on a plate and drizzle with the reserved balsamic syrup and olive oil. Sprinkle with salt. Serve.