I don’t know how much freedom you have to do these things. My current boss would flip out if I did this, but it is the most effective way I have seen to teach prepositions.
What you need:
2 objects (for this example, I’m going to use a toy dog and a toy house)
Paper
Red, Black and Green pencil.
Push all the desks out of the way. Have an area for the students to sit with you on the floor. The reason for this is because it’s a hands on thing, where you want the students to be focused on what you’re doing. You don’t want distractions of the books, desks, pencils, toys they brought in their bookbag that they try to sneak into class, etc.
Pick up the dog. “Sam (or whatever name), what is this?”
“A dog.”
Write on a strip of paper, in black, “a dog” and set it by the dog.
“What part of speech is ‘dog?’” (Answer: a noun)
Pick up the house. “Alex, what is this?”
“A house.”
Write on a strip of paper, in black, “a house” and set it by the house.
“What part of speech is ‘house?’” (Answer: a noun)
In red, right “put” and in green right “near.”
Now, put the sentence together. “Put the dog near the house. Susan, can you put the dog NEAR the house?”
“Good. Now, we’re going to change this green word. What other words can you think of?”
Have the students think of other words. Write them down and have them put the dog (in, under, on top of, next to, far from…you might even get “through”) the house.
After several suggestions and turns doing this, go back to the idea of the two nouns. “What part of speech is ‘dog?’” “What part of speech is ‘house?’”
“All these green words are what we call prepositions. A preposition is a word that tells us how these two nouns are related to each other.” (You may have to use different wording, depending on the students).
The next thing they can do is label sentences. Print up sentences that have prepositions in it. Make up a worksheet with simple sentences like:
The dog hid under the table.
Don’t throw any new or hard words in there, as the focus right now is to understand prespositions. With that, they can draw colored symbols over them to signify noun, verb and preposition.
In Montessori, the noun is symbolized by a big black triangle. The verb is symbolized by a red circle. The preposition, since it brings two nouns together, is symbolized by a green bridge. There is a reason why those colors and shapes are used and they don’t have to understand the reason right now. But it does make labeling the sentences much more enjoyable.
Here are the labels to the 9 basic parts of speech:

Top row, with colors since it doesn’t show them well on some:
Article (light blue), adjective (dark blue), noun (black)
Middle row:
Verb (red), adverb (orange), preposition (green)
Bottom row:
Interjection (tan, I think. I forget), Pronoun (purple), Conjunction (pink)
Hope that helps!
Matt
EDIT: Students can find their own sentences in books, magazines, etc. They can write them down and label them. They can also write them on strips of paper and tear it apart to see how else they can arrange the sentences so they make sense, keeping the labels with them. They’ll begin to instinctually see how the pattern of the language goes without having to think back on what rules to follow all the time.