You can do any hotel you want with that length of time. Near the city air terminal (which has bus service to/from the airport like every 10 minutes) are two hotels: the Inter-Continental and the Park Hyatt. These both are also close to subway stations that will take you most anywhere in town that you want to go to. I'd go to to Nandaemun market and wander around, but I haven't explored Seoul that much.