I did the DMZ tour once and it was a whole day tour, so you might as well forget about it. On a side note, I didn't think it was all that great.

Depending on where your hotel is, you may be able to do a lot on foot. Otherwise, taxis are plenty and rather cheap ("cheap" depending where you are from, obviously). The subway in Seoul is also cheap and it's clean, but I think it closes around midnight; it depends on the station, so you'd have to check to be sure. There are night markets in Seoul, so you could easily see and do things until 2 am. Seoul is quite safe. If you like spicy food you'll enjoy Korean food. Street food is safe, cheap, and delicious ... maybe not all, but most.