For mint syrup: Combine 1/2 cup sugar and 1/2 cup water in a pan. Heat it so that the sugar dissolves. Allow this to cool and add I cup of crushed mint leaves. Let it steep for 20 minutes. Strain and store the syrup. Use as desired.
Combine watermelon juice, lime juice, mint syrup and rum in a shaker and shake well.
Fill a popsicle mold three-quarters full with this mixture. Top up with club soda and stir to combine.
Freeze for about 2 hours or until mixture starts to solidify enough to hold a popsicle stick upright.
Insert popsicle sticks and let the rack stay overnight in your freezer. To release popsicles run hot water on the outside of the molds for a 2-3 seconds. Enjoy!