Our visit started in Beijing which is a short 1 hour and 45 min away from Seoul by air and spent 4 days there. I booked a reservation at the Beijing Guangyao Apartments/hotel via Booking.com and was happy to see it was an excellent choice. It is in the heart of the city, near the Chaoyangmen subway station and their two-room apartments were perfect for our larger-than-average family with 3 kids. It had a full kitchen, although we were surprised to see it did not have utensils but rather you had either to bring them or rent them. Traveling during late September was perfect since the air quality was OK for Beijing.
We are pretty practical in our travels, and used the subway in Beijing by in large. It is efficient, safe and cheap, (yet crowded), and if you download any number of subway apps for Beijing, you can easily plan your travels. I also had an off-line map of Beijing. Instead of buying a ticket every time, I just bought an IC card at the teller with a live-person at the station (costs only 20 Yuan for the card, and a minimum of 30 Yuan for use in trips), which you can reuse and refill with money when running low. For offline maps, my favorite is CityMaps2Go, which I've used for many, many trips in Asia, Europe and Australia. For useful on-demand offline tourist information, I use Triposo app on my ipad, or just brown many different websites ahead of time and get off-line copies by using instapaper app.
First day we visited the Temple of Heaven or Tian Tan, a Daoist temple dating to the early 1400s. It is large and takes about 2 hours to see at a not-in-depth pace.
Restaurants are plentiful in Beijing and food is very tasty. We avoid street vendors as a rule to avoid ruining a vacation from a GI infection, but otherwise food was great. Service in the other hand was often difficult due to the lack of English-speaking waitresses. So, by pointing at pictures, we got by just fine.
Second day was our visit to Tienanmen Square and the Forbidden City. The Forbidden City is amazing. It is a large series of imperial buildings that truly give you an idea of the the lifestyle enjoyed by previous emperors. Living in Korea, it is reminiscent of some of the palaces here, but at a much larger scale. We did not go to the Mausoleum for Mao, since I read on many sites it would likely be a long line and not much to see other than the obvious preserved Mao. Tienanmen Square is mostly a large plaza but does have a lot of locals which gives you some insight into their own local tourism. At multiple times during our stay in China, random people asked to take pictures with our kids, and we obliged since it was done purely out of curiosity.
After entering Forbidden City, it takes a good 2-4 hours to see the complex. I suggest starting in the south gate and end at the north gate, so you can then go on to Jingshan Park.
As mentioned, after seeing this beautiful place, try to get a little extra energy to walk up the steps in Jingshan Park (it is only about a 10 minute walk up) to get a superb view of whole Forbidden City from this high point and some of Beijing as well.
Afterwards, we hired a tuk tuk to take us to do shopping in the Wangfujing Street area and got some delicious late lunch there as well.
The other trip we did inside the Beijing area was to the Summer Palace. It does take about 45 min to get there by subway, but it was an easy ride. This is even larger than the Forbidden City but has a very different flavor since it has many trees and lakes and has a more country feeling to it. Unfortunately, the day we visited it was a rainy one, but it was still a great place to visit. We saw many local school children visiting with their school as well. Despite the bad weather, it was a fun day.
We could not leave Beijing without trying their famous Peking Duck. We went to the famous DaDong Restaurant (they have apparently 3 locations) and were not dissapointed with this tasty crispy dish.
Finally, in Beijing we also visited the Olympic Village, with the famous "Eagle's Nest" Stadium and the Aquatic Center.
We found in general in Beijing (and also in Shanghai) that the attention to customer service, not only to the tourist but the customer at large, is not as good as in some other regional countries. This was evident not only at hotels, but also at restaurants and tourist attractions. I guess there is still some "growth" needed in this part. Regardless, we found the average person to be very friendly and willing to try their English if possible. Beijing was still a great place to visit given its many historical sites to visit and great food to taste.