If you can afford wooden shelf units, face one one way and the other the other way and keep going, leaving a space to walk through if necessary. I did this for a room with two boys in it, and each one had two open spaces and two with a wall for cork board or posters. You can attach them with flat metal hardware so it can be taken down when needed.
I hope that helps.
My friend's daughter and boyfriend moved into a very small apartment. To split up the living room from the bedroom, the landlord used pallets as walls. You can usually get pallets free (ask around) if you decide to do this. It sounds weird, but it was really cool looking. :)