Xian is very nice! 3-4 days in Xian is more than enough, you can take day trips:

- Huashan -- it's really a nice mountain! I walked all 5 peaks on a day trip in bad weather even. including the plank walk --> not if you scared of heights that plank walk.

i did actually 4 days in Xian including 2 day trips outside. It depedns what you want to do. my exact itinerary: http://www.christravelblog.com/china...17-2014-05-05/

- Luoyang -- possible yes, but would stay overnight or combine with Zhengzhou and also see the Shaoling monastery which is int he middle. A taxi (if you can speak some chinese) coudl take you there for the full day at 350 RMB (cheap!).
like Xian - Luoyang - zehngzhou - Xian
I did it from Beijing and flying out to Shenyang. some idea on what to do more:
http://www.christravelblog.com/china...gmen-grottoes/

must warn you, the cities itself Luoyang is BORING. Zhengzhou also compared to other cities but it's better as Luoyang.
Xian has much more athmosphere.