This is as easy as can be and it's so good!
4 ripe Bananas (even if you have over ripe bananas, will do!) - peeled, sliced and frozen over two nights in an airtight container.
Walnuts or whatever nuts you like
A dash of Cinnamon
Take the bananas out of the freezer and thaw them for about 5 minutes. Put them in a blender and blend them till they are creamy. Add the walnuts and cinnamon and serve immediately. This won't keep after you blend it, but I don't think you'll have anything left over!
I have also read variations of this recipe adding peaches and other fruits. I'm yet to try those, mainly because I can't get over how good this one is.