Avocado is the secret to many a creamy smoothie, and makes a wonderful and equally tropical alternative to bananas for those who may not like the taste pr texture of that fruit. It's also a good source of heart-healthy fats and distinctive taste, which marries very well with fresh lime juice and basil (or mint, which would also work here). Be sure to use an avocado that is already ripened and soft enough for pureeing.