The new airport is awesome and they've really reduced the city's smog over the last five years' time. Also, in the Wangfujing district, they've impressively cleaned up the R.C. cathedral and it is really nice there now. I really liked visiting the Wangfujing Bookstore.
I was there in December - went to S.P., F.C., & T.S.. My daughter (who is Chinese) really enjoyed flying kites in T.S. and climbing the G.W. at Badaling.
Oops - I should clarify, I have two Chinese daughters, one joined us later in our China visit after we left Beijing..