Flying between Tokyo and Seoul is pretty cheap, and only takes a couple hours. If you are willing to devote a couple days to Korea, there are lots of great places you could go aside from the aforementioned places in Seoul. Gyoungju is the old capital of the country during the Shilla Dynasty, and takes about an hour and a half from Seoul via bullet train. Lots of cool historical stuff like Chumseungdae, the old kings burial ground (forgot the name), and Bulguksa, an massive old Buddhist temple with lots of great architecture and landscape. There's also Solak-san in the northeastern part of the country. It's a large mountain out in the country, and is a fantastic place to do some hiking any time of year. Of course, there's lots more too, but these are what I would consider the "must see" places if you only have a day or two.
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