Watermelon Margaritas. Watermelon is one of summer's best treats. Here's a thinking-outside-the-box idea for serving watermelon. Freeze cut up squares of watermelon and use as "ice cubes"! Serve in this yummy watermelon margarita (recipe) for adults, or as ice cubes in lemonade for kids. Perfect for 4th of July picnics or summertime BBQs and pool parties.