When you’re ready for a day trip out of Seoul, the country is your oyster.
Virtually all of mainland Korea is roundtrippable in one day, thanks to an excellent train and express bus system. While the locals often reserve their tickets ahead of time, the process is bit harder for foreigners to do. Your best bet is to head to a train station, where you can reserve tickets well ahead of time – in ENGLISH! – through an automated ticketing machine.