Boil together for 10 minutes 2 cups water and sugar. Add pineapple and lemon juice. To 1 cup of mixture add 1/3 cup ice water and freeze until firm. Cool remainder and strain over block of ice. Add ginger ale just before serving and serve in tall glasses with a ball of the pineapple sherbet in each glass. Garnish with sprigs of mint and maraschino cherries.