If you ever get the chance, hike up Mt Soraksan, about 3 hours north of Seoul by bus, on early autumn and just take in the colourful view of the countryside and mountainside. An easier way would be to ride a cable car to go up and walk the rest of the way up to the top. It can be quite windy at the peak and you can rest by sitting on the rocks/boulders around. Nice view, too, of the flame-coloured trees all around.
Fondest memory: The food! Authentic bibimbap [the trick is to mix everything well - the rice, veggies, pickled leaves, egg. Great during a cold winter's day and with hot sake. Then there's kalbi and it's great wrapped in lettuce leaves and topped with kimchi. During summertime, try a big bowl of cold buckwheat noodles - refreshing! Then good ol chapchae, the glass noodles with an attitude! :) Of course, all these must be taken with kimchi on the side. A good Korean meal is not just complete without it.
Mt.Sorak in Kangwon Province vividly shows the seanonal changes while differing its colours all through the year.
Fondest memory: While having a short trip to Mt. Sorak with my friends, one of them, eun-jung and I are walking down the narrow path in the mountain.
Favorite thing: try to climb Buk-Han Mountain. You will be amazing the tidy and nice feature of it. You can see whole Seoul top of there. I added its pictures to travellogue. Please stop by and see them ;)