Place a greased 6-inch springform pan on a double thickness of heavy-duty foil (about 12-inch square). Wrap foil securely around pan. Pour 1 in. water in a 6-quart slow cooker. Layer two 24-inch pieces of aluminum foil. Starting with a long side, fold up foil to create a 1-inch-wide strip; roll into a coil. Place in slow cooker to form a rack for the cheesecake.
In a small bowl, mix cracker crumbs, 1 tablespoon sugar, peel and ginger; stir in butter. Press onto bottom and about 1-inch up sides of prepared pan.
In a large bowl, beat cream cheese and remaining sugar until smooth. Beat in sour cream and grapefruit juice. Add eggs and beat on low speed just until combined.
Pour into crust. Place springform pan on top of coil. Cover slow-cooker with a double layer of paper towels; place lid securely over towels. Cook, covered, on high 2 hours. Do not remove lid; turn off slow cooker and let cheesecake stand, covered, in slow cooker 1 hour. Center of cheesecake will be just set and top will appear dull.
Remove springform pan from slow cooker; remove foil from pan. Cool cheesecake on a wire rack 1 hour. Loosen sides from pan with a knife. Refrigerate overnight, covering when completely cooled. Remove rim from pan.
Citrus Topping: Top cheesecake with orange and grapefruit sections (from half an orange and half a grapefruit) and kumquat slices. Add sugared cranberries (recipe below) if desired.
Chocolate & Pecan Topping: Melt 2 ounces semisweet chocolate with 1 teaspoon shortening and stir until smooth; drizzle over cheesecake. Top with glazed pecans.
Sugared Cranberry Topping: Lightly mist 1/2 cup fresh cranberries with water; toss with 2 tablespoons sugar. Place over cheesecake; top with orange peel curls.