Melt the butter and marshmallows in a microwave-safe bowl for 3 minutes, stirring halfway through. Add the mixture to 6 cups crispy rice cereal and stir until well combined. Transfer the mixture to the prepared pizza pan. Using buttered hands, press the treat mixture into the pan evenly and allow to set for 15 minutes. Cover the top of the pizza with whipped topping, leaving a 1/2-inch border. Top with a variety of fruits such as blueberries, raspberries, sliced kiwi and grapes.