Make the simple syrup: Combine granulated sugar with 1 cup of water in a small saucepan. Heat gently until the sugar dissolves completely (about 2 minutes). Let it cool.

Prepare the watermelon base: Puree half the cubed watermelon with 1 cup of cold water in a blender until smooth. Strain through a fine-mesh sieve into a large bowl, discarding solids. Repeat with the remaining watermelon and another cup of water.

Combine the lemonade base: To the bowl with the strained watermelon juice, add the remaining 2 cups of water, the freshly squeezed lemon juice, and the cooled sugar syrup. Whisk everything together. Taste and adjust sweetness by adding more water if needed. Store in a pitcher in the refrigerator.

Create the slushy: In a blender, combine the ice and the lemonade. Blend until you achieve a smooth, slushy consistency. Add fresh mint or basil now if you like, and blend briefly.

Serve immediately: Pour the slushy mixture into glasses. If desired, top with a sprig of mint/basil. Enjoy your incredibly refreshing drink!