Leftover watermelon-cucumber juice can be chilled and used up to 3 days. Watermelon and cucumber juices are extremely good for you and super refreshing on a hot summer day. These delicious fresh juices replace the sweet and sour mix found in most restaurant margaritas and the result is an addictive, thirst quenching cocktail. A splash of fresh orange juice or a little Cointreu adds another fruity element. If you have a whole watermelon and cucumber, cut small wedges of each to garnish the drink.