Seoul's Han River (Hangang) has been experiencing something of a renaissance in recent years, with extensive efforts being put towards ecological restoration, park improvements and trail-way extensions. And the result? Well, for cyclists, consider this your Tour de Korea course (sans the mountains). All told, there are 80 km of bike paths that stretch along the northern and southern banks of Hangang, winding under bridges, past urban architecture and through tall, grassy fields. Bike rentals are relatively cheap (around $3 USD/hr) and nearly ubiquitous, especially the closer you get towards the city center. This is also where you'll find the most popular sections of asphalt, like the 6 km southern route between Yeouido Island and the Banpo Bridge.
If you'd rather spend more time on the water and less time along side it, head to Ttukseom Park (one of several found along the Han). They've got everything from kayak rentals to windsurfing and water-skiing lessons available, including a sledding course for when that brisk Korean winter comes a'blowin' in. In other words, it can safely be considered your riverside go-to destination pretty much any time of the year. Check our links for more details.