We had moderately long layovers in Incheon airport going to and from Thailand. On the return trip we purposefully scheduled a 12 hour layover so we could spend some time in Seoul. The Incheon airport is great for transfers, one of the nicer airports we've been to. There is a lovely transit lounge with free showers (small fee for a towel), comfy chairs and a massage room. When we arrived early we were given vouchers for a free breakfast. Furthermore they offer free (!) trips into Seoul if you have a long enough transfer time. There are several options for this, we chose a trip to Gyeongbokgung palace and the Insadong shopping district.
It was about a 45 minute bus ride from the island the airport is located on into the center of Seoul which in itself is an interesting ride. This is the main gate to Gyeongbokgung.
The palace is located close to the center of Seoul with an impressive backdrop of the mountains that form Bukhansan National Park. There are many buildings that collectively make up the palace.
You aren't allowed into most of the buildings but there is good visibility inside. This is the throne room. This was a primary palace of the Joseon dynasty from around 1400 until it was burned in the late 1500s. The palace was eventually restored.
It is quite popular to rent traditional period Korean dress in order to visit the palace (in exchange for reduced or free admission). We saw hanbok rental shops just across the street from the main entrance. It's actually a nice touch as you get scenes like this.
There are a couple of snack shops just outside the palace walls. There is minimal shade in the palace itself but there are some nice shaded sections around the perimeter.
We had a guide with us as part of the Incheon airport tours. There are some signs (in English as well) but not very many. The buildings consist largely of residential quarters - separate for the king and queen. There is also a banquet hall, executive offices, a library and a lot of smaller residences for assorted staff.
The palace is right in the middle of Seoul. Just a short distance away is the shopping district of Insadong where we had lunch (also part of the tour). We didn't have a huge amount of free time to wander around here but hey, it's a free tour.