Step 1: In a blender, combine 1 whole pineapple (cored and sliced into chunks), the leaves from 5 stalks of cilantro (about 1/2 cup), the juice of 1 grapefruit and 4 cups water. Pulse until well-blended.
Step 2: With a fine mesh strainer (or strainer lined with cheese cloth), strain mixture into a pitcher or punch bowl. (Blending and straining can be done in two batches.) While I find the juice pleasantly balanced in sweetness, taste the agua fresca and add agave if you want it a little sweeter. Pour over ice.