During the Joseon Dynasty, Deoksugung Palace was one of the "Five Grand Palaces" built in Seoul. It is located near City Hall subway exits 1& 2, a busy intersection in the heart of the city. Admission is only 1,000 W, and the grounds are beautiful.
The neighboring area includes the historic Cheongdong church, the Cheongdong Theater and SEMA. I photographed all of the sites and will post/write in depth soon.
Sushi at an affordable little restaurant to the left of the main gate (heading in the direction of SEMA.)