China: Going to Be in Hangzhou for a few days and then have 5 nights in Beijing. Should I expand to other places?

I've looked over the guide for China and a couple of the most helpful threads but my situation is specific enough I thought I would post.

This September, I will be there with a group in Hangzhou, and then go to Beijing. Aside from landing there and flying out 6 days later, I am flexible. I've considered just skipping the Hangzhou --> Beijing flight and doing high-speed train to see more of the country but it would void my return trip. I'd love general advice and specific advice about:

• Should I pay $400 to fly to Xi'an? In general I want to keep costs low but I probably won't get another chance to go...


• Are there other places near Beijing worth an overnight?


• Is there a reason to book a package vs. a la carte tours of things like the Great Wall, Tiananmen Square, and the Forbidden Temple?


• How hard is it to use the subway? I'm decently travel-savvy and don't mind some (mis)adventure, but have almost no Chinese background - should I budget for taxis instead?

Thanks!