Fruit & fruit combination, on the other hand, is much safer, but make sure to adhere to the tips below: Sweet: Combine sweet fruits like watermelon, bananas, apples, cantaloupe, and plums. Acidic: You can mix blueberries, lemon, grapes, grapefruit, kiwis, and oranges, all of which are a great source of citric acid. See video for more: