In a small bowl, stir together the agave syrup and lime juice. (If using granulated sugar, whisk the liquid until the sugar is dissolved, about 30 seconds.) 2. Fill 4 glasses with 2 or 3 ice cubes each. Divide the strawberry and cucumber slices among the glasses. Add 1 tablespoon of the sweetened lime juice to each glass, top with seltzer and serve.