Two widely known attractions in Kashgar would be its Sunday Market and Abakh Khoja's Tomb. The latter, one of the holiest place in Xinjiang, houses the remains of Abakh Khoja, who was a powerful ruler of Kashgar, and the leader of the White Hat Sect of Islam. However, it is his grand-daughter, who was also entombed here (as was 71 other relatives/descendants), that has gained more fame than him today. She is supposed to be the Fragrance Concubine of the Qing emperor QianLong, and so most people know this attraction as Xiang Fei Mu (tomb)!
As for the Sunday Market, it is indeed an eye opener for many. However, certain parts of the market have succumbed to the tourist boom, and there is now a huge mall-like structure that houses scores of stalls selling the same stuff. Better to stick to the stalls outside, where you see all kinds of foodstuff and services being marketed. In particular, the livestock market should not be missed. Apparently, it has been moved further away and separated from the main market (possibly for hygiene reasons?). But it sure is far more interesting, as you can literally see the locals selecting/haggling over sheep, horses, cows and the like! Definitely one of the highlights of Kashgar!
The other highlight for me, is a walk into the Old Town of Kashgar. Here, away from the bustle of commerce and other tourists, you can see the real daily life of the locals. What's more, if you wander around at the right time, you can catch a glimpse of the children going/leaving school, and they are a delight to capture! They just love to have their pictures taken (and shown to them)! I could spend a day walking around just like that!
It certainly helps that Kashgar is also the start/end point of the Karakoram Highway, which attracts lots of travellers. But even if it's not, this city is worth a visit in its own right.