In a medium saucepan, simmer water and sugar together until sugar dissolves. Add next seven ingredients and simmer until tender, about 30 minutes.
Meanwhile, to assemble toppings, halve the plums and peaches. Remove pits, turn skin side up, and make a series of ⅛-inch-wide vertical cuts, leaving enough skin at the top to hold each half together. Place each half into a bowl, then spread slices out into a fan shape.
When soup is done cooking, remove cinnamon stick, divide into two batches, and puree in a blender (be careful–hot liquids can splatter), then strain. To serve, spoon soup into bowls, top each with fruit and a scoop of vanilla ice cream or fruit sorbet.