Google has had this tool called SketchUp, which is available both as a free version, and as a professional download package.
Both tools have varying levels of ease with which you can create conceptual design.
For someone getting started with their own mock - ups and concepts, this should be quite good if they feel comfortable with it. I have found that usually the problem I have with CAD tools is the amount of comfort I feel when I am around, using them.
So this tool may or may not be for you, and it is probably not for everyone, but is worth a try.
You can find a lot of detail about the products on the link (Click on Title).