Best software for game design in the conceptual phase

Hello everyone,

I'm a writer and aspiring video game designer and I'm looking for the best (preferably free) software for game design on the very basic conceptual level. I'm currently seeking employment at a local video game developer and i want to get some of my ideas down on paper so i have a portfolio to show. As I've already said I want to be a designer, not programmer or artist or anything like that so what I'm looking for is software that will allow me to illustrate my ideas in either 2d or 3d so i can have a visual representation. Unity was recommended to me but I'm not sure if that would be overkill or allow me to do just what i want. Any thoughts or ideas would be extremely helpful.