You don't have to get the silly individually wrapped marshmallows, by the way. Homeplus has a big bag of marshmallows for about 4,000 won.
My kids are probably going to make/decorate dipped Oreo cookies. I really wish we had the Trader Joe's peppermint ones though... yum. Super easy- Melt chocolate of your choice in a bowl in the microwave. Dip cookies in with a fork. Set on wax paper for them to harden. Cover in sprinkles while they're still a bit warm. (Homeplus has those, too.) Pop in the freezer for 20-30 minutes or so, then eat.