Cheong Gye Cheon used to be a road that has been restored into a stream flowing in the heart of Seoul. While the walkways on both sides of the stream weren't open when I visited, I walked along the sides of the road which now borders the stream from Dongdaemun to almost the western end near Gyeongbokgung.
This is quite an impressive restoration project. There are a lot of interesting design features along the stream, including rapids and a lot of tall grasses on both sides of the water.
More information : http://cgcfestival.seoul.go.kr/intro.jsp