Prepare a grill or grill pan to a very high heat. Place balsamic in a small saucepan. Turn the heat on medium, bring to a boil and reduce to a simmer until thickened and similar in texture to syrup, about five minutes. Let cool. Drizzle watermelon with a little bit of olive oil, season with a bit of salt and pepper. Grill VERY quickly, just getting a slight char, about ten seconds per side. Toss arugula with about a teaspoon or two of olive oil. Season with salt and pepper. Place watermelon squares on platter, top with feta, and another watermelon. Place about ½ cup of arugula on top of watermelon and drizzle with reduced balsamic vinegar.