Cut the watermelon into chunks and add it to a blender.
Blend until completely smooth, without adding any water. Watermelon already has plenty of liquid, and I like keeping that flavor as concentrated as possible.
Pour the blended watermelon through a fine-mesh strainer into a large bowl or pitcher.
Peel and prepare the clementines, pineapple, mango, papaya, melon, and bananas. Remove any seeds, tough cores, or rind, then chop the fruit into small, bite-sized pieces. I like keeping the pieces fairly small because tizana is meant to be both sipped and eaten with a spoon.
Add the pineapple, mango, papaya, melon, clementines, grapes and any extra watermelon pieces to the bowl with the watermelon juice. I usually leave the bananas out until closer to serving because they soften and darken faster than the other fruit.
Pour in the orange juice, grapefruit juice, and lime juice, then add the grenadine.
Stir everything gently until well combined. Taste it before adding any extra sugar. Depending on how ripe and sweet your fruit is, you may not need any at all.
Cover the bowl and refrigerate the tizana for about 2 hours so everything gets nice and cold and the flavors have time to come together.
Right before serving, gently stir in the sliced bananas. I like adding crushed ice directly to the glasses instead of the whole bowl so the tizana stays cold without getting watered down.
Spoon plenty of fruit and juice into each glass and serve with a spoon, a wide straw, or both.
Enjoy!