Trip to South Korea 26.April - 10May (Seoul)

Hello Community,

I am planning to finally see Korea, end of April - beginning of May. Of course I have plenty of info on the internet, still could not find something like: "hidden" places I should not miss.
I want to start from Seoul & go south till Jeju, and from there probably take the ferry or any ship to Okinawa-Japan.

Could you share some tips & tricks about traveling in South Korea?

Many , many thanks for your support
Andreea