I had a great visit to Seoul recently. Compared to many countries in Asia there isn't a lot of English spoken, though. My one suggestion is if you are a foodie, learn to read the hangul alphabet otherwise you will not be able to eat at most restaurants. The alphabet is incredibly easy to learn, I just made some flashcards and had it down in a day. This only works if you know the name of some Korean dishes, but those are also very easy to pick up.
The subways are very easy to get around. I also did the hop on hop off bus and thought it was worth the price. A lot of the museums in town are really interesting. War musuem was good and I don't usually go in for that sort of thing. My favorite was the kimchi museum. Some of the larger temples have tours in English, but you really don't need a tour. I also took a cooking class at the
Institute for Royal Cuisine that was great. If you like cooking, I'd highly recommend taking a class somewhere because Korean food is amazing!
DMZ tour was B-O-R-I-N-G. It really felt like a huge American propaganda piece (I went with the USO) and I left feeling ripped off and wondering why I had woken up so early on my holiday.