Never tried it either with syrup or without an ice-cream maker, but if you want to make a granita - that is, a sort of frozen slush - you can do that by diluting the syrup to taste, then adding in a tablespoon or two of vodka. This will slow down the freezing process enough to avoid having big ice-crystals. Pour the liquid into a shallow container and figure on stirring it periodically through the freezing process. I use a Donvier maker, the kind with the pre-chilled container, and that's basically how it works.
Here is a wonderful recipe for sorbetto al melone (cantaloupe sorbet). My notes say it's from Bon Appétit, May 1997. This fragrant sorbet is served in hollowed-out melons decorated with raspberries and elegant chocolate curls. Cancaloupe melon is recommended, but if unavailable other melon varieties may be used; adjust the sugar depending on the sweetness of the melon.
2/3 cup sugar
1/2 cup water
3 cups 1-inch pieces peeled seeded cantaloupe (about 1/2 cantaloupe)
Combine sugar and water in medium saucepan. Stir over medium heat until sugar dissolves. Bring to boil. Transfer to 11x7x2-inch glass dish and chill until cold, about 2 hours. Puree cantaloupe in blender until smooth. Add to sugar syrup in dish and stir until well blended. Freeze until almost firm, stirring occasionally, at least 3 hours or overnight. Transfer cantaloupe mixture to large bowl. Using electric mixer, beat until fluffy. Return to freezer and freeze until firm (do not stir), at least 3 hours or overnight. Sorbet can be prepared 3 days ahead. Cover and keep frozen. Serves 6.