This was my response (belated) to MessedUp, who asked the board if he should skip Xian on his trip. I just wanted to share more widely some insights from my honeymoon in China in October, 2004. This board was a great resource for my wife and I, and maybe some of our thoughts can be of help to other travelers:

Contrary to the other posters, I would recommend that you do skip Xian. My wife and I visited China for our honeymoon in October, 2004 (Shanghai 2 days; Suzhou 1 day; Beijing 5 days [Jinshanling to Simatai Great Wall - fantastic]; Xian 2 days; Yangshuo 4 days; Hong Kong 3 days). We did it on our own (no tour), and it was a fascinating trip. As for Xian, we thoroughly enjoyed the Muslim Quarter and its night market, biking on the city walls was a pleasant way to spend the afternoon (and to look down into unexplored sections of the city), and the Terrracotta Warriors are an impressive sight. HOWEVER, it was by far the most polluted city we visited in a country with significant pollution problems. I ended up with a lung infection, and slept in a chair for the last week of the trip (I'm 34 and in decent shape; some antibiotics cleared up the infection when we got home). Furthermore, we found the dining options in Xian to be very poor (we're not gourmets) and two different cabbies there blatantly tried to rip us off (we took cabs in every city we visited); we actually had to jump out of one cab. Admittedly, our knowledge of Chinese history is extremely limited, and maybe we did not appreciate all the cultural and historical sights in and around Xian, but we're fairly seasoned and hardy travelers, and the pollution in Xian was simply overbearing. Moreover, while the Terracotta Warriors are tremendous feats of artistry, I found them somewhat overhyped; basically, what you've seen in the pictures is what you get -- rows of clay figures under a huge industrial hangar. The awesomeness of it fades somewhat quickly, and if that's the sole reason you're visiting Xian, I don't believe its worth the trip. My wife and I had strongly considered Lijiang/Dali before going, and if we had to do it over again, we would have skipped Xian and gone there.

As for some recommendations:
(1) Take the subways in Beijing: while not extensive, they are a fun, inexpensive, and easy way to get around and experience life as a native.
(2) Explore the Back Lakes in Beijing -- join in the ball room dance lessons on Saturday nights, get a dirt-cheap 20-minute body rub, and visit the restaurants recommended in the guide books.
(3) Tour the Hutongs
(4) If you need to choose, visit the Summer Palace over the Temple of Heaven.
(5) Jinshanling to Simatai Great Wall - stunning, and highlight of the trip; absolutely worth the long ride, and you get to see a nice bit of the countryside along the way. We hired a private driver through one of the hostels (even though we stayed at the Grand Hyatt) for I believe $90 for the day, and it worked out great -- he brought us to the airport when we were done.
(6) Yangshou over Guilin - touristy, yet not the same level of trap as Guilin, and it offers all sorts of great cruising, hiking and biking options. Although you'll fly into Guilin, take the local shuttle vans from Guilin to Yangshou - another great day-in-the-life experience. And look up Sam (of Uncle Sam's) if you need a guide on a bike trip.
(7) Longsheng Rice Terraces: they're worth the rough 3-4 hour ride from Yangshou; beautiful, and you will be introduced to a unique ethnic community based around the terraces (I forget their name).
(8) Be prepared to get lost a few times, and just laugh about it (we ended up in some pretty comical situations).

Anyway, these are just my opinions. No matter what you do, you'll have a great time, and Xian definitely has some things to offer. Enjoy.