I went during the Holy Week (April 12-15), and my friend and I were looking for a place that still has a full bloom cherry blossom trees. our second itineray on that day is a park in Jamsil station-- we didn't know the place first, until we learned that it's the Seokchosun.
We really had an amazing day on this lake since there were still a lot of cherry blossom trees blooming that day. On the other side and very accessible to the lake is the Lotte Amusement Park wherein it's very good if you are with your friends or kids.