I saw this really difficult but fun map called SkyBlock on the forums. Its basically a block with a tree on it and very limited resources. The challenges are very difficult and time consuming. I had to start over 2-3 times because I didn't calculate my resources right to complete all challenges. Here is the official tread on the forums: www.minecraftforum.net/topic/600254-surv-skyblock/
I hope you enjoy it.

Additional Notes

All the credit for the map itself goes Noobcrew. I just played his/their map.