Unfortunately for where you want to go there isn't really an 'off the beaten track' type of thing you can do.
I think Xian is worth it for the Warriors, Old walled city and a number of monuments and buildings in the city.
And another near Xian a couple of hours drive is Huashan which is a full day trip but I think it was good. Plenty of people though. Hard climb though doable in 1 1/2 hours.
huashan - Google æœå°‹
You can do Xian in a three day, two night trip or two days two nights and then fly or catch a train in the morning to Beijing.
Pingyao is a good option but a little difficult to get to but it may have changed when i was there in 2005.
Yangshuo is worth it. Get to Guilin and then get the boatride down to Yangshuo. Not much in Guilin really so get in there in the afternoon or night and then get the boat down the river to Yangshuo.
Shanghai is worth a visit if you haven't been before but not much there. Beijing is good. Tiananmen obviously, Forbidden City and Hutongs behind the wall and the Great Wall definitely.
There are a number of places for the wall. The main tourist attraction is Badaling. Though I have never been there it's the main tourist wall area which is very busy. It's also been reconstructed so you can see the original form.
I have been to Simatai which is alot less people but still great. It's untouched alot and hasn't been recreated.
simatai - Google æœå°‹
The other area which I recommend is Mutianyu. it's also relatively untouched but more dramatic and very steep. A number of hotels are around now and can stay close by. We rode a horse up to the bottom of the wall.
mutianyu - Google æœå°‹
We drove to both these locations so couldn't recommend a travel agent but with you are staying at the YHA they can organise something relatively cheaper.
If you haven't been to China then Beijing (4 days, 3 nights), Shanghai (2 days, 1 night), Xian (2 days, 2 nights), Guilin/Yangshuo (3 days, 2 nights), Pingyao (2 days, 1 night) and probably Chengdu (though I haven't been there).
But if you left out Shanghai you wouldn't miss much.