The cave has a closing window; the riverbank is more flexible but exposed to weather. Keep the second stop short enough to drop.
Set the cave entry first
Gosu Cave is listed year-round. Korea Tourism Organization gives hours of 09:00–17:00 and adult/teen/child fares of KRW 11,000/7,000/5,000. A Korean Heritage Routes page lists last entry at 17:00 and describes about 940 metres of visitor passage with a 30-metre elevation change. Enter well before the cutoff; there are steps and damp surfaces. Check the operator notice before travelling.
Add a riverbank stop if daylight allows
Dodamsambong is listed open year-round from 09:00–18:00 with free admission. The three pillars—Janggunbong, Cheobong and Cheopbong—rise from the Namhangang, with Samdojeong Pavilion on Janggunbong. Parking is listed at KRW 3,000 for small vehicles and vans, KRW 6,000 for buses. See the Dodamsambong listing.
In warm months, put the longer outdoor pause earlier or later in the day; in winter, protect daylight and shorten the river stop. Rain, wind or poor visibility can make the bank less comfortable, so the cave alone is a complete visit. No single transit schedule connects these stops in the official listings; check the actual transfer before fixing the order.