For the spicy syrup: Put the sugar, jalapeno and 1/2 cup water in a pan and bring to a boil, stirring until the sugar has dissolved. Reduce the heat and simmer until the liquid has reduced to a thin syrup, about 10 minutes. Remove the jalapeno. Let the syrup cool, then add the lime zest and juice.
For the cowboy fruit salad: Add the strawberries, melon and green and red grapes to a bowl, pour over all of the syrup and toss. Sprinkle over the mint leaves.