You might try using Google SketchUp. It's a free download of a 3D Modeling program, although it might be more sophisticated than what you're looking for.
http://google-sketchup.en.softonic.com/Forgive me if any of this info is incorrect, it's been a while since I've used the program and don't have it downloaded on my current computer to check, but I believe you can upload a pdf or jpg of a groundplan to use as a base, or draw in your own, and then import 3D models of furniture and objects from their stock and move it around, rotate, stack, etc. You can also use the groundplan as a template to build a 3D version of the stage or wing space, if you want to get super fancy. It was fairly easy to figure out the controls and what each button did, how to get something to move a certain way, all of that was pretty intuitive and there are some great online tutorials. Again, that's the recollection I have of the program from a scene design class I took in undergrad, so the new version might have different specs. It was easier than Vectorworks, that's for sure.