Activity: Set up an obstacle course using couch cushions and chairs.  Tell your child to go under the table, over the cushion and around the chair.  You can expand to include other furniture and more prepositions.

Suggested Script: We are going to play a game.  First you have to go under the table, then go over the cushion and then go around the chair.  (You may have to demonstrate.)

Other Activities: Take turns giving directions.  Let your child set up an obstacle course.  Encourage him/her to use prepositions when giving directions to you for the course that he/she has created.