Have you checked with the Korea forum for suggestions? Is this your first time in Korea and what initially attracted you to Seoul?

This post-trip write-up from a somewhat recent traveler has good summaries of a lot of the tourist areas: Post Trip Notes

If you're interested in checking out a theme park within Seoul, there's Lotte World. There's both an indoor and outdoor portion and its close to the Lotte World Tower complex.

There's also Samsung's version of Disneyland called Everland, which is about 30-40min south of Seoul.

One important tip to remember, Google Maps does not work reliably in South Korea. Download either KakaoMap or Naver Maps to get accurate directions around Seoul.