You can get to this green and mountainous area just outside of Seoul easily on the blue subway line. The park is a welcome change from inner city Seoul and has plenty to keep the whlole family entertained. There is a zoo, a botanical garden, an art museum, a ski-lift ride, a lake, a theme park; the list goes on. We went to the zoo and it was pretty impressive. It was all fairly open and spacious, and you can pay about 50p for a ticket to a dolphin show which is good fun. You don't get stung too much for food and drink inside either which is refreshing. You pay a little more than usual but it still adds up to a cheap day out in the countryside, in the sun.
This is Korea's foremost zoo, part of Seoul Land. This is certainly enough for a day, so I created a separate tip for it. It's comparable to zoos in the US or Europe.