While not a CAD program, I've found that Apple's Keynote, the presentation software works surprisingly well for drawing objects and then piecing them together to resemble actual mechanical drawings. If you don't need the precision and the ability to generate construction drawings per se, then this app might be worth a shot.
For other mockups, in particular an iPad app I'm working on, I've found Omnigraffle ideal. It has an extensive library of objects and you can create your own. So, that sense it is similar to Keynote in its applicability for testing design ideas and manipulating objects.
Frankly, I've found Google SketchUp Pro more than frustrating. It is not a drafting program but rather a object rendering program that makes creation tedious and annoying. Others have become comfortable with it, but I'm not in that crowd.
If you are interested in how Keynote objects can be developed into a design, my sketches are in the Forum at:
http://www.tnttt.com/viewtopic.php?t=39401&highlight=