Keep your vocabulary simple. Younger children can learn the concept of prepositions without having to learn the terminology for it. Use simple phrases, like "today we're going to learn about all the places you can go," or "let's talk about what you can do," rather than telling the students you're going to give them a grammar lesson on prepositions.
Go on a bear hunt. This classic preschool favorite is chock-full of prepositions. Go through this song with your students, and then talk to them after about all the different things they could and couldn't do on their bear hunt (for example, "Can't go over it, can't go under it, etc.). While you sing the song, you can have students make gestures for the various prepositional phrases as they continue their bear hunt.
Use magnetic manipulatives. Get a dry erase board or other metallic surface. Students should be able to spread out and use the board in small or large groups. Give each student two magnets, or pictures mounted onto magnetic strips. Tell them to put one picture/magnet over, under, around, through, and so forth in relation to the other magnet. For examples: a tiger and a hoop, a dog and a doghouse, a bear and a cave (to go with a bear hunt theme).
Incorporate music. Several popular songs might help your students remember the vocabulary they learn. A few examples include "The Bear Went Over the Mountain," "Somewhere Over the Rainbow," and "Hickory Dickory Dock."
Incorporate artwork. Allow students the opportunity to express themselves creatively by making story books. Give each student five sheets of paper, then tell them a word. Students will draw a picture of someone doing that word. For older students, they can make their own picture. For younger students, you may want to provide them with cutout characters they can simply glue onto the paper instead. For example, you say the word "under," and children draw a picture of a cat hiding under a bed.