It's not every day that you can see tremendous gardens full of beautiful lotus flowers, but if you make your way out to Yangsu Station on the Jungang Line, you'll be able to all that and more. The entrance price for the park is 3,000 won which seems high to get into a park, but when you leave, you can get free ice cream or fresh produce with your ticket, so its definitely worth it.
I don't really need to give much explaination here, the photos say enough. It's a beautiful park and I suggest people find the time to make it out of Seoul to check it out. It's about 50 minutes from Wangsimni station. I don't recommend that people with cars drive out here, traffic on the weekends tends to be atrocious on a good day and just plain not moving on a bad day, like today. This would also be a nice place for a picnic too, lots of open grassy spaces which are not too common in Seoul!