2 Days between Boryeong Mud Fest and seoul

For culture, you might want to look into Gongju and/or Buyeo. They've each got a couple of sights worth checking out. Buyeo is the smaller (and quieter) of the two. There should be fairly regular buses from Boryeong to each of the cities.

For nature, you could head to Daejeon to go for a hike in Gyeryongsan National Park (I think you can also access the west side of the park from Gongju, though I've never done that myself). It has a few hikes that are okay. You could also take a look at Chilgapsan Provincial Park, which is maybe 20-30 km east of Boryeong, but I haven't made it there yet, so I can't comment on it.

Jason

I just got back from a weekend in Gongju and I highly recommend it. It wasn't crowded, even on a major holiday weekend. The mountain fortress is really nice and pleasant to walk around and there's a nice small temple there. We also visited Magoksa, which is a large temple complex in the woods. It was lovely and, once again, not terribly crowded.

The only reservation I may have about Gongju is getting around. While I'm sure there's buses, I don't know how regularly they run. Taxis may be expensive as distances can be far (it's larger than Seoul in area and has lots of rural areas). I drove my own car there so I didn't have any experience with buses/taxis.

Awesome - thanks everyone!