little science fair

I’m wondering if you ladies have any ideas for me:  Our homeschool group is planning a “science fair” in October.  We’re a small-ish group (11 families); the oldest children are in 5th grade.  

Basically, the rules are it has to be science-oriented.  And that’s it.  So, I’m wondering: for my daughters (4th grade, almost 2nd, Kindy, and 3yo) what could we do??? We’re using AiG’s Technology and Inventions book for science this year, so it’d be nice to have something related to that  – printing press, telegraph, telephone, tv, computer, etc.  But some of that seems so advanced.  

My other question is how in-depth or detailed to go.  I’m imagining poster boards with pictures and perhaps some manipulatives or hands-on sort of thing.  I don’t know!  Maybe another way of presenting it….when the children present things to the group, they usually stand at the front of the group and recite or read whatever.  I’m guessing the presentation can be done however we want.

Maybe I need a better defined box before I can start thinking out of the box, you know?

Any ideas? Thanks!