an immersion blender and a cocktail umbrella (or whatever fun garnish you'd like)
Cut off a small piece of the bottom of the watermelon so that it can sit without rolling. Set the watermelon in a bowl to stabilize it and catch any drips/overflow. Using an immersion blender (but not turning it on yet), press it gently into the top of the watermelon and twist to score the rind. Then twist and push harder, working in a circle to pop out the top "plug" of rind. If your immersion blender head is not sharp, you may have to use a knife to help get the "plug" out.
Now for the fun part: Press the immersion blender into the flesh and pulse to start liquefying it. Make sure to scrape down the sides and be careful not to go through the bottom. The process can be a little messy, so wear an apron. Once the flesh is mostly liquid, top it off with lemonade and a shot of vodka or rum if desired.
Garnish with lemon slices, mint sprigs and a cocktail umbrella or other fun garnish.