Remove the zest from 2 lemons in wide strips with a vegetable peeler, then juice enough lemons to make 2 cups. Refrigerate the juice.
Combine the sugar, lemon zest and 1 1/2 cups water in a medium saucepan. Bring to a simmer and cook, whisking occasionally, until the sugar dissolves, about 2 minutes. Transfer to a pitcher and refrigerate until cold, about 1 hour.
Remove the lemon zest from the syrup. Pour 3 cups cold water and the lemon juice into the pitcher and stir. Remove 1 cup of the lemonade to a bowl and add 3 cups water. Divide the watered-down lemonade mixture between 2 ice cube trays and freeze until solid, about 2 hours. Refrigerate the pitcher of lemonade until ready to serve.
Add some lemonade ice cubes to the pitcher along with a few lemon slices. Serve in cups with the remaining lemonade ice cubes.