For adventurers who want to gain a more immediate sense of what is lost (and what remains) with the passage of centuries upon centuries in a barren landscape, you have the Jiaohe Ruins. There is something more immediate about seeing what's left of a 2300 year old city built from earth and then abandoned centuries ago that transcends the pottery you'll see encased in glass in a museum somewhere.
An important stop on the great Silk Road and possibly the most well-preserved ruins of its kind, Jiaohe is located on the cliffs of an inlet between two rivers in what is today's Xinjiang region. There are numerous buildings that still stand, including the remains of a monastery, a large tower, and what may have been government offices, although you'll be reminded more of the Star Wars planet of Tatooine than an earthly civilization.
Far to the northwest and closer to the border of Mongolia than to any of the major cities of China, a visit to the Jiaohe Ruins, which are one of the largest of their kind in the world, is definitely an off-the-beaten-path experience. Getting there will undoubtedly be tough, there aren't even public buses from nearby Turpan. Resourceful adventurers may rent a motorcycle or horse-drawn gharry, although a taxi ride shouldn't cost you any more than 40 rmb.
Within a few hours drive, you'll also find the ruins of another ancient city, Gaochang, which served as big stop on the Silk Road trade route as well. Check them both out! For more useful information on things like transport and when to go, browse the included website.