What are some typical snack foods, lunch foods that we can try there? Where are good places for these? What should we avoid?
Any particular dinner place we can try in Beijing?

Dadong has great duck. Reviews on Tripadvisor

We stayed at the Park Plaza. Right behind the hotel and down to the corner there are two things we thoroughly enjoyed. For breakfast there is a hole in the wall place that serves a delicious wrap. It's got egg, black bean, and a bunch of other things. To die for. Also near there is a dumpling shop. We went back several times. Dadong is right across the street from this hotel.

I don't know about the current edition, but the edition of Frommer's Guide to China that I used had an outstanding section on foods from each region of the country, with the name and description in English and the name also in pinyin and Chinese characters. Well worth finding!

And an enthusiastic 2nd on the recommendation to go to Da Dong for Peking Duck. Be sure to reserve in advance.

We also enjoyed dadong for duck, but be aware that the cost was not inexpensive. The braised eggplant dish we had there was fabulous. The menu is extensive and a bit confusing. The atmosphere is very nice and upscale.