The Han River parks are where Seoul actually relaxes, and the flat bike path along the south bank is the best cheap afternoon in the city. This loop — Yeouido to Banpo and back — takes about three hours with stops and costs under 15,000 KRW.
Getting a Bike
Two options at Yeouinaru Station (Line 5, exit 2 or 3). The public Seoul Bike (Ttareungyi) docks are everywhere — 1,000 KRW per hour with the app, though the bikes are heavy one-speeds. For a longer ride I rent from the private shops under the park's overpasses: 3,000–5,000 KRW per hour for a decent hybrid, helmet included if you ask. Bring ID; some shops hold it as a deposit.
The Ride East
From Yeouido Hangang Park, follow the riverside path east. You will pass under Mapo and Wonhyo bridges — the path is fully separated from traffic the whole way. Around Nodeul Island there is a good rest stop with a convenience store and river-view seating. Continue to Banpo Hangang Park; the whole stretch is about 8 km and almost completely flat.
Banpo Bridge and the Fountain
Banpo's Moonlight Rainbow Fountain runs April through October, usually at 19:30, 20:00, 20:30, and 21:00 on weekends — check the schedule before you go because it cancels in wind or rain. The show is 20 minutes of water and colored lights off the side of the bridge. Grab fried chicken from one of the delivery carts that roam the lawn (yes, they deliver to your picnic mat — just tell them your mat's location) and watch from the grass.
The Return
Ride back the way you came, or cross Banpo Bridge's bike lane to the north bank for a different view and return via Yeouido's north side. Evening rides in summer are the move — the path is lit, the heat breaks, and half of Seoul is out doing the same thing. Return the bike by 22:00 and you are steps from the subway.