I taught 6th grade last year... We did a Pictionary-type game for prepositions. I had cards with different prepositions words on each one. I started as the "illustrator" by choosing a card without anyone seeing it. Then, I illustrated the preposition on the board.
For example, if the preposition was "under" I could draw a table with an object under it. I would be able to use only hand gestures to elicit answers from the students. This could also be a quick way to "teach" the concept of prepositions showing place
List the words on the board as they are identified by students....
As the kids recognized the prepositions and the purpose for them, I would have them state the prepositional phrase (i.e. "under the table").
They would then write sentences with the phrases.... then paragraphs....
Pictures from magazines can also be used by the students to identify and create sentences with prepositions.
"Prepostion Pictionary" was also played by my students taking turns at the board and pulling a preposition card. If the kids aren't too crazy, you could have them "act out" the preposition
Also, I've had my students go on grammar hunts with magazines, books, and newspapers to find prepositions, verb phrases direct and indirect objects, punctuation, etc. Making it a race game with a timer works well with the 6th graders.