Last week I went to Odusan Unification Observatory in Paju, only one hour away from Seoul by bus. It's the closest to the North Korea I've been, 3200m, but I don't think it's the most interesting place. It was really really interesting because it's like a museum and you can see many objects from North Korea daily life but you can't really see North Korea. It's just empty, I wish I could show you what I saw from the telescope but on the other side, it looked like a fake ghost town.
To get there, take bus 200 from Hapjeong, get down at the Unification Park station, it's pretty deserted, don't panic and go to the parking lot: there is probably the shuttle bus waiting, it will take you for free to the observatory. :)