Skyblock is a very popular map for Minecraft on PC. It has hundreds of thousands of downloads there and is expected to reach the same numbers on Pocket Edition soon.
Far up in the sky is a floating island built. It’s only a few blocks in size with a tree and a chest on top. From here you have to survive as long as you can on the skyblock. By using the items found in the chest and carefully using the tree in a clever way (such as destroying some of the leaves to try and find apples) and such it is possible.
See how long you can make it on the island! Perhaps you manage to expand it and create a self sustainable city in the future.