The Hyeopjae and Ssangyong caves were formed by the eruption of Mt.Halla, and are considered unique because there are stalagmite and stalactites which aren't often found in lava caves. Fossil remains of seashells have led scientists to believe that the caves were below sea level when they were formed by flowing lava millions of years ago. These two caves are part of a network of more than 20 caves in the area -- most which are not open to the public. Surprisingly, the caves seemed to be one of the least visited attractions in the park (at least while we were there), which worked well for us.
We explored the park for a couple hours, talked to a lot of birds and lizards, read a totally ridiculous cave legend about some doctor that played with a bead in the cave (?) and met a lady who was actually a fox, walked around the underground pillar of knowledge, stopped for drinks, bought a few small souvenirs. and toured the bonsai gardens. For the most part, it was an enjoyable way to spend a morning before hitting the beach for the remainder of the day.