Any ideas for cheap, interesting prop/set design pieces for a live action role playing game?

Live Action Game will include Vampires, werewolves, Fae, and mages. Sounds incredibly geeky I know and maybe it is, but it is also a blast!

This will be for a semi-large live action game of 100 - 200+ people and we're trying to come up with some cool set/prop designs for general room/stage atmosphere but have a small budget, limited tools when it comes to saws or power tools (though we have more than 6 months before the day they are needed so we have time and plenty of hands to help).  We do have several great artists working with us and a lot of creative crafty people but are only beginning to brainstorm possible ideas so I figured I'd ask for some suggestions from some of the web's cleverest DIYers.

This is much like setting a stage for a theatre performance. Our convention space is pretty large and we have multiple large (think ballroom sized) rooms available to put our set in.

Other requirements for pieces:
Fairly light  so we can move the items pretty easy (the game will be taking place in a convention space that we do not own).
Ideally pieces could be broken down some for easy transportation and storage
Would be great if some of the pieces had some versatility so we could use them in multiple venues (ie main structure, that can have things that can be switched out for other things is awesome for some of the pieces, like something covered with a canvas painted scene that can be switched out for a different canvas scene or similar idea)
Nothing that is permanet or requires drilling into a wall/floor etc as the space is not ours and is rented
Nothing that is on fire or requires fire.
Nothing too complex for the average person to put together and take down
Safety is a concern

I would be interested in hearing any ideas or being pointed to websites for inspiration or instructions on creating these kinds of stage/prop pieces.

Thanks in advance for any ideas!