Add a short layover in Korea if possible to experience the food, culture and urban life while hopping around Asia. 1 or 2 nights in Seoul can allow you to see a lot.
Seoul airport offers a free tour of the DMZ on Tuesdays, which I paid around $50 for from central Seoul. They offer different tours each day at set times, check out their website and sign up well in advance if possible.
Visit one of their hot springs 'resorts' in the city, which often times allows you to spend the night for a small additional fee for a cultural immersion experience. The hot springs I stayed at near Seoul's main train station had numbers hot spring pools, hot tubs with green tea, hot stone rooms, oxygen rooms, gaming room, small cinema and the top floor was filled with dorm beds to sleep there on the cheap.
Lotte World is a fun way to spend the day if you have kids or like roller coasters. Most of the amusement park is located inside, with a few other roller coasters and rides located around the Castle. The entrance fee is much more reasonably priced than other amusement parks and there are lots of different food options inside priced basically the same as outside the amusement park, so didn't feel there was any price gouging going on.