1. Tea
If they have the habit of drinking tea, it is better to send tea. Tieguanyin, Oolong tea, Pu'er tea, etc.
But you must ask, there are also many Koreans who don't have the habit of drinking tea.
2. Green Tea Cookies
Food made with green tea is very popular in Korea these years (because people are getting more and more health conscious). Green tea cookies are not too sweet and are enjoyed by both men and women.
3. Tianjin Clay Man Zhang
Koreans are very fond of handicrafts with exquisite workmanship.
(Don't buy imitation works made in other places)