In a medium bowl, combine the rice, almonds, cinnamon stick and 2 1/2 cups hot tap water. Allow the mixture to cool, then cover and refrigerate overnight. Pour the mixture into a blender, add agave (start with 1/3 cup, you can add more later) and blend on high for several minutes, until the mixture is as smooth as possible. Add one cup of cold water and blend for 10 seconds. Place a large metal sieve over a large bowl. Line the sieve with cheesecloth (or use a nut milk bag or clean paint straining bag, found at hardware stores). Pour the mixture through slowly, stirring as you pour. Press on the solids with a spoon to extract as much liquid as possible. Lift the cheesecloth/bag and squeeze the rest of the liquid out. Discard the remaining dryish pulp. Pour the mixture into a pitcher and stir in the last cup of water. Pour into glasses filled with ice and serve.