I went there decade ago.
Qinghai Lake (esp. the bird island) is a must, but not the best timing to see birds.
Tar Lamasery (Ta'er si Temple) is another must, where you see everything in Tibetan Buddhism.
Not far from this temple, it's a grass land where you can do hiking and horse riding, meeting Tibetans, they have great home-made yoghourt. But September is late for green grass though.
It also might be very cold, better prepare down coats. I visited there during late September and early October, people there wore thick sagums already.
Early September could be cold, it's a province where snows mostly visits in October.
Ta'er si Temple is in the southwest, very near Tibet, further south, no! No mood to explore, cuz didnt bring enough clothes and felt bad about the food there..