I have wanted to go to Cusco since I was eighteen years old and in my first year of university. It is phenomenally beautiful, Inca walls prop up Spanish colonial mansions and churches in a clash of cultures unlike anywhere else I know of. The city is higher than most mountain peaks in Europe and within stones throw of some of the most impressive megalithic ruins I've ever seen.
Sacsayhuaman in particular has enormous blocks of granite that must way hundreds if not thousands of tons fitted together in an irregular pattern with a level of precision that is astonishing. There are theories as to how these blocks were placed but given that Inca history doesn't have the written record of many of the other great historic civilisations the truth is unclear and even includes some “aliens did it” rubbish.
We also visited the fortress and village of Ollantaytanbo, the circular terraces of Maras Moray and Machu Picchu.
Thousands of words have been written about Machu Picchu and it really is one of the modern wonders of the world. I can't really add to what a google image search will give you, however we did the Machu Picchu mountain climb which I could not find much information on. I would not recommend it if you are afraid of heights, there are sheer drops next to narrow uneven steps. It is tough and should only be attempted if you are a confident hill climber. It is around 2 hours to the summit and gets steeper as you get higher. We also missed out on much of the views as there was too much cloud cover, that said I'm glad we finished it.
If your going to Machu Picchu you have to visit Ollantaytanbo, it's got stunning ruins that can be seen in a day and the village itself retains it's Inca character. We booked in for two nights and ended up staying for four.
Moray is absolutely worth a visit if your visiting the Sacred Valley. Again it looks like a flying saucer landing site and it's actual purpose is unknown. Terraces are used extensively in the Sacred Valley for agriculture and wikipedia tells me some people think the concentric circles create a micro climate to test what environment is the best for growing crops.
None of these sites disappointed, we're currently in Arequipa which is not as high but very relaxed. We'll be heading into Bolivia to see the salt flats of Uyuni and then on to the Atacama desert where I hope to reach in time for the new moon. Chile is one of the best spots for astronomy in the world so I am phenomenally excited about seeing the milky way if we can get there on time.