My boys' project was on the Periodic Table -- they put together their own periodic table and chose a title ("The Periodic Table -- One Step at a Time"), then selected 10 facts to share about the periodic table. They struggled with coming up with basic enough facts (these kids know a LOT about this topic from their own reading on the subject), but wound up with a really nice project to share with others. Zoo Boy spent most of his time at the Science Fair standing at their exhibit and answering questions about it.
J, on the other hand, loved visiting the other kids' exhibits and filling out his "passport" (the kids got stamps at each exhibit for answering a couple questions about the exhibit). He earned a science pin for his hard work, and he enjoyed seeing what the other kids had done for projects. This was the first time we have been to a science fair, so this was a learning experience for all of us.
The boys accept certificates of participation and ribbons at a ceremony at the end of the Science Fair.
It was a really great experience all around, and gave a nice science-y feeling to our rhythms for the month preceding the Science Fair as they worked on their presentation board. I'm hoping this becomes an annual event!