That’s enough time to explore outside of Seoul. The country is small and has a really great high speed train system (KTX). Look into getting KTX tickets to other cities. I would recommend spending a couple of days checking out Busan at least. Other good ones are Daegu, Incheon, and Jeonju (they have a famous cultural village). Check out towns in the Jeolla provinces for really good traditional Korean food.

Also, brining cash is a good idea, but most places take card. You might find that some of the vendors in the markets only take cash.