In a small saucepan, bring sugar and water to a boil. Remove from heat and let cool. Transfer to an 8-inch square dish and mix with juices of oranges and lemons, crushed pineapple, and bananas. Freeze 3 hours (or up to overnight). Remove from freezer about 20 minutes before serving (mixture should be slightly slushy). If desired, mix with some lemon-lime soda, and garnish with mint sprigs.