It seems like American theme parks have existed forever. Disneyland came to be in 1955, and parks like Coney Island have been around over 100 years. But other countries don't have such a rich heritage.
Seoul Land opened in 1988, just in time for the Olympics. It's located a bit outside of town, in Gwacheon, but it's easily accessed via the city's subway network.
In the summer, the park opens its pool, and in the winter they have snow sledding.
I'd really like to find time to get back here on a future visit when it's warmer, if crowds remain pretty light at least.
Seoul Land is found at Seoul Grand Park, a large public area that also includes Seoul Zoo, an art museum, and a large lake. Take Line 4 to Seoul Grand Park Station, and use Exit 2. Head straight along the wide walkway in the parking lot. When you reach the large building, you can walk left around the loop road to get to the park (it's a bit of a walk) or take the Elephant train (for a fee) - the train goes counter-clockwise so you'll stop at the zoo first.
The park usually opens at 9:30, closing sometime between 17:30 and 22:00 depending on the season and day of the week. Admission only is 20,000 won (about $20) and an all-day ride pass with admission is 36,000 won ($36).
There's a small discount for those arriving after 5 PM; if you're planning on staying only a few hours and don't need daylight for photos that could be a good idea. But it's been my experience that crowds pick up when discount admission begins.