If you're looking for a lesson that requires virtually no preparation but has the students engaged the entire class, you should try this 20 questions lesson.

1) Go over what a YES/NO QUESTION is. I explained that there are five general types:
    "Do you..."
    "Is your (it)..."
    "Have you ever..."
    "Can you..."
    "Are you..."

With each of these I give lots of examples. Do you like kimchi? Is it raining outside? Have you ever been to a foreign country? Can you play a musical instrument? Are you an only child?

I also call on individuals in the class. I ask students a question with one of these question types. Its fun if you make them personal/silly. For example, "Are you handsome?" or "Do you like Girls Generation?" (I teach at an all-boys high school so these usually go over well). To get them speaking a little more do follow up questions like "Which girl in Girls Generation is your favorite?" etc.

2) Tell students you will now give them the ANSWERS and they must formulate the QUESTION. Examples are:
    Yes. I like soccer. (Do you like soccer?)
    No. I am not going to China next year. (Are you going to China?)
    Yes. I want to be Pikachu. (Do you want to be Pikachu?)

3) Explain the 20 questions game (at least one student always knows it). Explain that the person in the front thinks of something (anything they want) and then everybody else has to ask only YES/NO questions to try and guess what the thing is. I usually go first so that they understand how to play. With higher level classes I pick something hard (once I did The Pyramids in Egypt and they actually guessed it...) and with lower level you can make it something easier (usually something in the classroom).  After one round, start picking students to come up and be the thinker.

No one falls asleep during this class and its a good review for most students. Good luck!
   

I like this lesson. Or at least I HOPE I like it. I'm going to try it out soon. I thought I'd try it tomorrow, but suddenly my classes were canceled to allow the students time to prepare for the mid-terms.

Anyway, I'm going to use this as a good excuse to review the difference between yes/no and 네/아니오. We'll see how that goes!

Thanks for sharing and reminding me of this game man. This should work well with some of my daily classes as well with my after school class.

For the question about it lasting for a 50 min, maybe it could. This should help with adding some entertainment at the tail end of the class, if need be.

http://www.20q.net/

It has multiple language support too, if the students want to see if they can outsmart the computer in Korean as well. You'll have to work in a way that it teaches English though, if you want to be proper.

I have played this where I put them in teams. I usually tell them it has to be a person, place, or thing (so they don't just think of a random word in their vocabulary). I did it with an extra class and I made them into teams. The team that guessed got a point. People on teams had to take turns asking questions, etc. It was fun, the kids got into it. I made them write down their word first so they couldn't cheat and so I could make sure the students answered the "yes/no" questions correctly.

For some reason http://www.20q.net/ is blocked by my school? I'm always surprised by the things i'm allowed to look at and the things I'm not!

this idea is so great i cant believe i never thought of it before............. ...

I've played this with my classes before and they love it.

If you want, you can give the answering student a picture to work with. It's always hard to come up with enough people/things, so here's what I used for a game once. It lasted about 30 minutes. This might at least get some people started who want to use it for a whole lesson.

Thank you for this. Classes that don't use powerpoint are usually the best. I will try it out on Monday, and I am sure that it will work. 

I am sure it will work!!!!! :D

Most of my students enjoyed this game, but some of them showed no interest. Those students felt the game was too easy. To make it more interesting for them, I had them teach part of it to the class. It got them interested in the lesson too!