One good day trip that you can try in South Korea that is quite out of ordinary is a combination of Gangchon Rail Bike and late lunch of Grilled Eel near Ungilsan station.
The Gangchon rail bike features an old railway turned into a bike park. You can pedal along the lovely Bukhangang River and old tunnels in the area. Based from my experience, it is a tiring but fun experience- imagine, you need to pedal a bike for around 50 minutes ( with a partner), but in exchange, you get to see lovely mountain and river side sceneries. Of course you don’t have to worry about intersections and all since this track is not used anymore so you’re safe. One more thing is that if you don’t know how to bike, it should be ok since you’ll be riding a sidecar which can technically balance on its own.
The starting point of the bike track is in Gimyujeong station, exit 1. Right after exiting the station, you will see the Gangchon Rail Park signboard rightaway with the numerous 2-seater and 4-seater bikes. Just show your voucher (which you can purchase online in Indiway) in the entrance gate and the staff will give you the tickets in exchange. A 2-seater bike costs 25,000 korean won, while a 4-seater bike costs 35,000 korean won. Pretty affordable since these costs are group-charged already.
Before you start the trip, the staff will inform you about the guidelines and instructions (how to pedal, maintaining distance with the bike in front of you, etc). After that, you are on your own- and you can pedal your way through the tracks!
Gangchon Rail Park also had some gimmicks plotted inside the tunnels. Some tunnels will be pitch black inside, with colored lights being played in tune of famous K-pop songs like Gangnam style, etc. You might have a hard time taking a video though since you cannot completely stop (it will cause delay to the other bikes).
The whole experience lasted for around 60 minutes, with a short break (10-15 minutes) in between. The brief station stop has a mini coffee shop (where you can by coffee, Tteokbokki, chips, etc) and public toilets that you can use. Make sure to maximize this break as you won’t be allowed to stop in any other areas of the track.