How can I temporarily partition a large open space to create another room?
You can get divider screens that are made of several reasonably lightweight panels connected together with hinges (like the IKEA RISÖR). Something like that could be used to divide the room. I've also seen people use freestanding shelf units to divide a large room, that are open on both sides (like the IKEA EXPEDIT), and then fill the shelves with books, decorative vases/bowls, and collectibles.

Neither option really creates a wall, but you're looking for something more temporary. Anything tall enough to go to the ceiling will either need to be really wide for stability, or attach to the ceiling, which I don't think you want to do.

Note: The linked products are used as examples of the type of furniture I'm writing about. They aren't meant as recommendations for specific products.