Yeouido (Island), Seoul
I love going to yeouido and spending a few hours at the park. It's a great place.
You can go rent a bike from Yeouido park in neer the national assembley. It cost around 3000W for an hour.
You can ride around yeouido for a few hours, then go sit by the edge of the Han river and sit and cool off with a beer. Yeouidos great.
Best to do on a hot sunny day!