This mocktail is perfect for a summer get-together. Fresh fruit is great for garnish, but for syrups, use thawed frozen fruit. Picked when it was at its peak, frozen fruit is full of flavor and often cheaper.
In a small saucepan, bring the sugar, chopped rosemary, orange zest, and 1/2 cup water to a simmer over medium heat, stirring until the sugar dissolves. Remove from heat. Steep for 10 minutes; strain.
In a blender, puree the thawed frozen berries with their juices; strain.
Divide the rosemary syrup and berry puree among 4 glasses filled with ice. Top with seltzer. Garnish with rosemary sprigs, orange slices, and fresh berries.