I had my students first do a picture search--like a hidden picture thing and they have to say where they hidden objects are. You can turn this into a game.

I also had students draw their own picture of a house, then describe it to another student who has to draw it--the tree is NEXT TO the door. or whatever. they can draw a picture of anything really.

Of course these are both mainly for location prepositions

there is a Prepositions lesson on this website with the Where the Hell is Matt? video.  I did that with great success doing my prepositions lesson first then the video and then a worksheet to describe where Matt is dancing and pictures of each frame.  Matt is dancing next to the crabs, Matt is dancing under the waves, etc. 

the video is just 2-3 minutes long so it should not seem too time filling.

Those are all really great. I'll try to pick them to her. She does want to try and do something different, something that isn't a cut and dry prepositions activity. That's why we were so excited about the treasure hunt. Everyone's ideas have really helped our racked brains, though. Thanks!

A couple short things:

Tape something (a flash card or a picture) under the chairs, and have the students check under their chairs at some point during the class. "What's under your chair?" Slightly off target language, but they'll think it's funny.

Set up a tray with objects like a cup, a small box, a bag (see through is best), and put objects in and around it. Give the students a few moments to see it, and then ask them where the objects are. You can do this a couple of ways, by arranging it behind the desk and lifting it up, or by blocking their view with a big piece of poster board.

Along the lines of Where's Waldo, there are the I-Spy books from Jean Marzello and Walter Wick. If you can get scans of their books, those are good photos to work with. Here's their website, with online games: http://www.scholastic.com/ispy/games/index.htm

You can also put things around the room, but you'll have to make it visible from the desks if the room is that crowded.

There's a dinosaur preposition powerpoint somewhere on the website, I've used that before. Edit: Found it - http://www.waygook.org/index.php?topic=47168.0

sorry this isnt an idea, just a tip: Use it, or don't :)

I don't really think you need to go over board and prepare new things that you wouldnt usually do in class. If you look at the way most of these teachers teach in English, they lack confidence...some...

I had student teachers come to my classes for 2 weeks, i did nothing special, just the usual book work and review sort of stuff, and the fact that i had confidence when teaching impressed them, and well...the class was a hyper class in general so they responded well.

So don't feel to presured into impressing them...just having confidence and not generally being fake...if you know what i mean.. works well.

Also, most of my classes weren't powerpoint based, it usually involved the students talking as much as they can in a game way, either writing exercises, were they speak wha tthey wrote, or role-plays with props, (parents love seeing their kids talk and how good they are, my principal wanted to see that too and was quite happy when the kids were able to repond to me in english, so it's not just you doing all the talking all of the time but just giving direction here and there.)

Sorry if you didnt need any advice...just puting it out there :)