Travel suggestions out of Mokpo area??

My son and daughter-in-law are teaching on an island south of Mokpo. My wife and I taught last year in China and in February we plan to visit and travel together. I have been checking out travel prices and destinations for the February holiday and find that it is very pricey to fly anywhere. I assume that you have to fly out of Seoul, since Jeju and Kwangju seem to only have domestic flights. We will be coming into Korea on the train and ferry from Osaka to Busan. Apparently we can take a ferry from Incheon to Weihai on mainland China but all the flights I've checked are very pricey: eg. to Hanoi, to Bangkok to Kathmandu. Lonely Planet mentions that you can link up with a courier and get discounted flights by accompanying parcels through customs! Has anybody tried this?? Does anyone have any suggestions??? We are open to any and all suggestions.