Step 1In a large bowl, combine the rice and cinnamon sticks. Bring 6 cups of water to a boil, then pour the hot water over the rice and cinnamon. Cover and allow to soak overnight.
Step 2In a blender, puree the rice mixture (including the cinnamon) with the agave nectar until smooth; this will need to be done in batches. Strain the mixture through a mesh strainer lined with a triple layer of cheesecloth (or a chinois). Stir in 6 cups cold water, or adjust consistency to taste. Adjust the sweetness to taste with additional agave nectar if desired.
Step 3Just before serving, stir the horchata vigorously. Serve over ice and garnish, if desired, with a few chopped toasted pecans and a sprinkle of diced cantaloupe. This will keep for 3 days, refrigerated.