For the salad, toss cucumber, strawberries, blueberries, and blackberries in a mixing bowl.
Drizzle with honey and lemon juice. Stir gently to combine.
For the coffee cake, preheat the oven to 350°F (175°C).
In a bowl, mix flour, sugar, baking powder, baking soda, and salt.
In another bowl, combine sour cream, butter, egg, coffee, and vanilla.
Pour wet ingredients into dry ingredients and mix until smooth.
Fold in walnuts if using.
Pour batter into a greased 9-inch cake pan.
Bake for 35 minutes or until a toothpick comes out clean.
Serve the salad topped with fresh mint and the warm coffee cake on the side.