Seoul to Dandong

Hi im trying to arrange a trip to see a friend in Dandong for Chinese New Year.
Now I will be in Beijing for 5 nights was hoping to go somewhere else for a few nights then onto the city I want to go. I was thinking South Korea but the transportation and flights seem all over the place and the from return from Seoul to the city very long due to stop overs. If not Seoul somewhere else in China. I was thinking Shanghai but again all flights are booked up due to the festival