I suggest you change things around a bit and plan on winding up in HK during the "Labor Day" week. Just buy a ticket into PEK returning from HK and connect the dots.

You can do as much of the Beijing-Kunming schlep over land as your heart desires, which might be interesting for you and would give a chance to stop off in some smaller cities. But, given your time constraints, I suppose you might to cap train/bus travel at an arbitrary number like 24 hours.

Anyway, between Beijing and Kunming, in addition to Xi'an, you could easily hit up Jiuzhaigou and Chengdu. And once you get to Kunming, you can move on to Lijiang, Shangri-La, and Dali.

I would advise against over planning because I've done trips like yours in the past and it's nice to be able to add/delete destinations on the fly. Also, by not planning too much, you will be less suscpectible to the ripple effect that a single canceled flight or sold out train could impose upon you.