Huanu Picchu is the mountain that you see in the back of every classic picture of Machu Picchu. There is a moderately step trail that goes up the side of the mountain to the very top. From the top you can get great views of the Lost City and see the famous outline of the condor. If you have the time to make the hike, it is definitely worth it.
If you are interested in doing this hike make sure you have done some training before you get to Peru. If you are in shape, the hike will be much more pleasant. Take plenty of water and make sure that you have good, broken-in hiking boots.