Step 1: Prepare the simple syrup if not already made. Combine equal parts sugar and water in a small saucepan. Heat over medium heat, stirring until the sugar dissolves completely. Remove from heat and let cool completely. This can be done in advance and stored in the refrigerator.
Step 2: In a large pitcher or a sturdy bowl, combine the fresh mixed berries and fresh mint leaves. Gently muddle them together using a muddler or the back of a spoon. You want to release the juices from the berries and the oils from the mint, but don't overdo it to avoid bitterness.
Step 3: Add the lime juice and the cooled simple syrup to the muddled berries and mint. Stir well to combine all the flavors.
Step 4: Fill four serving glasses with ice. Strain the berry-mint mixture into the glasses, distributing the liquid evenly. You can press the solids with the back of a spoon to extract more juice, or leave some pulp for a rustic look.
Step 5: Top each glass with chilled sparkling water or club soda. Stir gently to mix.
Step 6: Garnish each sparkling berry mint summer mocktail with a few fresh mint leaves, a couple of fresh berries, and a lime slice or wedge. Serve immediately and enjoy your refreshing summer mocktail recipes!