We really enjoyed seeing the palaces, the changing of the guard and the temple in Seoul. The DMZ and the Third Infiltration Tunnel were interesting. the Fish Market in Busan and the APEC were very impressive.
Riding the KTX to Gyeongiu and then from Busan to Seoul was very nice and gave us a chance to see the countryside.
On our last day we took a trip on our own to Nami Island and Petite France and throughly enjoyed that as well navigating the Metro through Seoul to Yongsan Station and taking the ITX train to Gapyeon and then a bus and finally a ferry to Nami Island. If it would have been a little warmer we would have taken the Zip Line.