The trading card game idea is one of the best I've come across yet. It really forces the students to use the language and they were engaged the whole time.
I wanted to keep their interest so I made a card game with all sorts of famous Koreans and cartoon characters. Just print out the Word documents and paste to construction paper, then cut out. Here's my directions:
Superhero Card Game! Teacher creates several “superhero” cards with varying abilities (strength, age, speed, height). Students battle each other with cards. Students battle by laying one card on the table face down. Then they play rock-paper-scissors and the winner chooses the question (Which (one) is older? Which (one) is taller? Which (one) is smarter? Which (one) is faster?). Let the students battle each other and before the game gets stale sit them down and count their cards.