The Great Wall of China is one of the greatest man-made structures in the history of humanity. It stretches over 5500 miles of China, and it was built starting as early as 7th century BC. It is hard to fathom how difficult it must have been to build this long of a structure without modern technology.
When you see the Great Wall in person, you are only visiting small sections of the wall that are easily accessible. It seems to go on forever, yet you are not even seeing close to 1% of it all. It is definitely one of the coolest sites that you can see anywhere in the world.
I visited the Mutianyu section of the Great Wall. I am not sure which is the most common and easily accessible section to visit, but Mutianyu is a good choice. Mutianyu is great because you take a cable car/chair lift up to the initial section. If you do not enjoy heights, I would NOT recommend the chair lift, as you are extremely high above the ground in an unenclosed chair lift (basically a ski lift). The cable car is a much safer option (or feels like it), as it is completely enclosed. Still, either way, you will get a great view of the Wall on your way up.
The best part of going to Mutianyu is that you can take a toboggan down the Great Wall. The toboggan ride is super fun (it seemed safe, but a little old so be careful) and is an incredible way to end your visit to the Great Wall. It is like a giant slide down, and you have the ability to control your speed (up to a certain extent).
China has plenty of history, and The Great Wall is a big part of it. The Great Wall is very old, but it is still in relatively good condition (especially in the parts popular with tourists). Even if you have seen pictures and videos of it, it is hard to put the scale of it into perspective until you see it first-hand.