Only 5 days - top picks for Seoul and countryside?

I will be in South Korea for a mere 5 days in mid-September. A friend lives in Seoul so I will be based there. I'd like to get the flavor of the city, and also explore some of the idyllic countryside and small towns I've been reading about. It's a priority to visit at least a couple of Buddhist temples. I welcome any suggestions (and tips!) for making the most of a short but sure-to-be amazing adventures.

Im interested in pretty much the exact same thing, only a month later...any advice would be phenomenal. thanks

Seoul - Gyeongbuk or Changdeok Palaces, Namsan Tower (preferably so you get sunset and then the view at night), Jogyesa Temple, Namdaemun Market, Insadong, maybe Yeouido for a view of the Han River, maybe Gangnam/Apgujeong for some posh nightlife. Also on the edge of the city, Bukhansan National Park is beautiful.

A jaunt using the excellent bus system to Seoraksan National park might be good. Seorak has some pretty incredible sights, temples and the most picturesque of South Korea's "spirit mountains" The town of sokcho also has a number of beaches. It is about 2.5 hours on a bus to get there from Seoul. I second the Bukhansan National Park reccomendation too.

Hello there!
If you are in Korea for only 4 days, you'll be very busy trying to catch all the good places
Since its only a few days, I advise you to plan out each day with each area's surrounding destinations.
It is also important that you know exactly how you will get everywhere so that you don't waste time trying to figure how to get to your next destination. I recommend using the subway for traveling to practically all the places you wish to go at least in the vicinity of Seoul. Buses seem to be more of a hassle for me.

Now for the destinations.
Since you will be in Seoul, you probably should use 2 days to tour the city.
Some good places I recommend are the N-tower, Myeondong, KyungBokGung Palace, Insadong, Dongdaemun, Hongdae, etc.
Rather than me tossing you names, you'll probably be better off visiting this website: http://english.visitkorea.or.kr/enu/index.kto

I'll find some good pages for you so that you can get started.
*Instructions on using the subway: http://english.visitkorea.or.kr/enu/TR/TR_EN_5_1_4.jsp
*Seoul 1 Day Tour of Must-See Closely Located Places: http://english.visitkorea.or.kr/enu/SI/SI_EN_3_6.jsp?cid=261738
*If you enjoy amusement parks, here is one in the city which you can spend about half a day visiting: http://english.visitkorea.or.kr/enu/SI/SI_EN_3_1_1_1.jsp?cid=264152

Hope you find this info helpful! Have a great trip in Korea~ :)