2 weeks ago I had pretty good success with a Prepositions lesson that I whipped up on a whim. All my lessons run short every single time, without fail (except with my 3 extra-dumb classes where they STILL don't get it by the time the bell rings), so keep that in mind if you want to use this!
I start with writing "In, On, Over, Under, Beside, Behind, Infront of" on the board, and then telling the kids to teach me those words. The first few times I can usually stretch this out for at least 5 minutes, as I'm still learning them, and not knowing what they said...by the 5th or 6h class, I have to pretend to get it wrong to drawl time on. I write the korean in a different colour.
in case they don't know a word, here they are in order:
??, ??, ??,

, ??, ??, ??
Make sure they add the extra ae to A-rae-ae (Under)... "ae" means "at", if you have a smart class and think they can handle that extra info...or maybe they already know.
Since they don't distinguish between On and Over in Korean, you have to explain this quickly. I just said "the chalk is ON the desk. The Chalk is OVER the desk." with actions, and repeated twice. Every class for some reason repeated me the second time without me even asking. (as if I could be so lucky every other time I ever wanted them to repeat!)
<5 mins>
So then stand on the desk at the front.
"I am ON the desk" (it also gets the attention back on me, as I've already lost some kids after only 5 mins)
"You are ON the chair"
(still looking at the same kid, but pointing to another) "He is ON the chair."
Then I write that formula on the board
I am ===== ____________
You are ==== ____________
He is ==== _____________
(=== mean a different colour blank...matching the Korean words' colour)
Then I get some kids to answer "Where are you?" and I point to a random word, and they must use that word to tell me where they are.
Eg: "I am BESIDE gyeong dam."
I usually start ith examples.
Make sure you practice Infront of and Behind the most - they always get them mixed up.
I switch it up for the smarter kids and ask where I am, or where 'he' is? and make them give 2 descriptions. (Eg: He is infront of Min Cheol, and beside the window")
<10 -15 mins>
(On a side-note, if you're so daring... They ALWAYS mix up He and She when I ask them those qustions, so I like to differentiate by getting back up on the desk, grabbing my man-parts and yelling "HE!" as I give them a jiggle. Then, cupping my man-boobs, I yell "SHE!" And give them a shake too ...Repeat two or three times for maximum effect and laughter. The kids absolutely LOVE it)
Then I hand out my work-sheet. I've altared it to include 3 examples...but maybe you don't want to be so generous hahaha
They know what it is right away, I don't have to tell them. what I DO have to tell them seventeen frigging times is that they need to Write me one sentence for each item, describing where it is. (as if my examples were easy and formulaic enough to figure that out)
Eg: The heart is under the branch.
The Cup is on the man's knee.
The Spoon is beside the tree.
I don't usually distingiush too much between right and wrong answers as long as they wrote me a full sentence.
This actually takes 20 - 30 mins depending on the level.
I usually prod them alone by writing on the board "The _____ is ===== the _______."
As the kids start to get wriley, I start making them stand up and read 2 answers. I give out cnady for this, so they fight to want to talk.
<5 mins>
For smart classes, I found it only took 5 - 10 mins to do the worksheet. With them, I played the following game.
I start by pointing to a student and making a statement (Eg: You are under the light.) That student must then point to any other student and repeat the statement (regardless of truth). And that student must point to another, repeating again. Etc. Tell them at any time, they may change the statement to a question (Eg: ARE YOU under the light?).
-- if the student is too slow on replying, they lose, and you start again.
-- if thy answer incorrectly, they loose and you start again.
--if they answer correctly and quickly: candy & THEY start with a NEW statement, pointing at another student. Repeat.
I just draw this out to the end of class. Once they get the hang of it, they really enjoy it. It's basically just a drinking game with no booze hahaha