sorry, i just saw this. you're probably already done with it, but here goes for next time or for the next person in a similar situation.
you'll burn a couple hours each way going to/from the airport. full tourist mode? depending on what time your layover is for, you could get a Panmunjom tour, they'll take you to the JSA, a watchpost, and the infiltration tunnels. do the USO version -
http://affiliates.uso.org/Korea/default ... tentid=347they also do city tours and a folk village tour. the skyline boat tour might be kinda interesting if you dig architecture, i found most korean buildings to be kinda bland though. skip insadong, it's just a tourist infested market with half the shops carrying the same tchotchkes as each other. i'd suggest dongdaemoon for clothes shopping and yongsa for electronics.
the palace is kinda cool too but it's just a reconstruction of the one the japs burned down when they annexed the country (same goes for pretty much any histcorical building except the buddhist monasteries). however it is also right next to current seat of the SK gov't, the blue house. the buddhist temples are the originals for the most part though. there are 4 big ones in the seoul area. bongwonsa temple has ceremonies and performances on the 1st and 3rd weekends of the month, and jogyesa is where the lantern festival is held to celebrate buddha's birthday.
CJ is right, the korean bbq is probably the only cuisine that's worth writing home about. The live octopus is a cool little delicacy, but you won't get any that's decent in seoul, gotta head down south to busan for that.
that should probably get you through 12 hours if you just wanna do some touristy stuff.