#8--Do a little research on the following:
--Wutaishan area. This is between Datong and Pingyao but off to the east. Might be worth a couple of nights. A little bit harder to get around though.
--Luoyang. East of Xi'an and Huashan, but very close by bullet train. A day and a night would cover it.
--Tianshui. West of Xi'an. This has the Maijishan Grottoes. It's doable as a daytrip from Xi'an by standard train, if you're willing to get up early and return at night. Tianshui city is rather dumpy but the grottoes are out of the city (public bus runs out there cheap from train station).
--Chengde northeast of Beijing, usually done by bus (3 hrs) with parts of Day 1, overnight, morning of Day 2 then return to BJ.
On time in Beijing, I hear that you're not wanting to be rushed, but I'd start out with 4 days and plan on backloading another 3-4 more at the end.