To make fruit concentrate: place watermelon, cucumber, basil, and lime in a pitcher or large jar and smash with a wooden spoon to release their juices. Stir in water. Let the mixture infuse in the fridge overnight, then strain.
To make simple syrup: combine water and sugar (or honey) in a small saucepan over medium heat. Simmer, stirring occasionally, until completely dissolved. Let cool.
To make soda: for each serving of soda, combine ¼ cup fruit concentrate, 1 tsp. simple syrup and ¾ cup seltzer water in a glass. Serve with ice if desired