I chose a ten planet model, incorporating Pluto and Eris, with a modified scale (see step 1). Distances, for reasons that you know doubt understand, are not to scale. My model is not mechanical, although the planets can be rotated individually.
For more information and plans to build a true mechanical orrery, check out Clayton Boyer's beautiful designs here.
Edit: Thank you to everyone who voted for me in the Celestron Space Contest! As you might be able to guess, this is still a work in progress. I will post the final pictures after it is completely finished. Thanks!
Step 1: Calculations
I set Jupiter's size at 7 inches (diameter) and scaled the other planets relative to it. Be aware that seven inches is a lot bigger than it sounds and a lot heavier than you think (Jupiter weights ~3 lbs). Many of my later design constraints came from Jupiter and Saturn's weight.
The terrestrial planets and earth's moon are magnified by a factor of 5 in order for them to be visible (In the photo below, Pluto at ~0.6 inches in diameter is approximately the correct size for Earth without the 5x magnification). The sun is essentially not to scale, for reasons that are unlikely to become clear at the moment.
The calculations below do not include Eris. Reports vary as to whether it is in fact significantly larger than Pluto or only slightly larger. I chose the smaller value for the radius as it is the most recent. All the data is from Nasa. The scale was calculated in excel and modeled in photoshop.