The Great Wall in China is one of the great architectural wonders of mankind. Build in the 15th and 16th century it was meant to protect the northern borders against Mongolian threats. With a size of 2400km it isn't really something that you would like to trek as a whole (it's mostly destroyed anyway) but you can join organized multi-day hikes - a good idea actually because the day-tourist hot-spots are extremely crowed. The great wall is a very different experience when hiking a more remote and barely restored portion. We did a 2 day variant from Xiangshui to Mutianyu where the wall follows some very scenic mountain crests. It is a fairly easy walk with no technical difficulties but it can be very steep in places so a decent level of fitness makes this a more enjoyable experience. As you may imagine the wall is very exposed to the elements so make sure to take a hat, sunscreen and rain gear just in case. On this tour variant you stay in a guesthouse just under the wall in Jiankou so all you need is a daypack with a little bit of spare clothing and a lunch box.