Make the Mixed Berry Simple Syrup
Combine the berries, water, and sugar over medium-low heat and bring to a simmer. Let it cook for about 5 minutes until the sugar dissolves. Add the simple syrup to the jar of a blender and puree on high. Strain into a bowl to remove seeds, and cool.
Fill a glass with crushed ice. Add 2 tablespoons mixed berry syrup and rum, then top it off with cola and additional berries. Serve immediately.