|1000 yard solar system|
The general outline is:
ASP's Project Astro Universe at Your Fingertips, section activityD-7 "The Thousand-Yard Model (or the Earth as a Peppercorn)"
This one is found in a number of places on the 'net, for example (this is a more involved version of the above) http://www.noao.edu/education/peppercorn/pcmain.html
See Activity D-7.
time: 30 - 60 minutes
I've found that working out the Sun and Mercury as a class, and then everyone works individually to fill in the rest of the table.
Explain that they will be divided in to 2 person teams. (if there are more teams, you can add other objects, like the moons of Jupiter, or where a spacecraft currently is). Try to encourage more than recital of facts - some groups have done songs and skits.
The teacher will make time available for them to develop their material - typically the second visit is about a week later.
time: 90 minutes
Start at the sun...if the Sun is visible, when you get to Earth, they can cover the sun with their finger (real and the model one) to see it does scale.
Here's pictures from one of the years:
I have a picture prepared that the teacher can display/print out afterwards that shows them where they've been.