Gongju in Chungcheongnam-do really should get more attention.
There's a Guinness World Record Buddha head at Waujeongsa Temple in Yongin.
Seoul Sky Observatory should be on everyone's must see list.
The DDP in Seoul is an architectural marvel that should not be missed.
If there's one thing you do in Seoul - let it be renting a hanbok and wandering a Joseon Dynasty Palace.