Dd8 seemed to need guidance on picking a subject, so I asked her (for my own culinary purposes) to see what effect soaking whole wheat grains in different liquids would have on their subsequent sprouting. She made observations over the course of several days (including weighing, which wasn't very informative due to the water involved in sprouting), and we found out that vinegar and salt water will pretty much kill off the sprouts. Tap water yielded good sprouts, and Brita-filtered tap water was even more productive.
Dd6 had to be dissuaded from mashing a variety of kitchen items (flour, sugar, bananas, etc.) together and "seeing what happens". She eventually came up with (on her own, bless her stubborn little heart) a project where she blew a whistle at a flag with various objects placed in front of the whistle. She learned that air flow can be hard to predict. :)
I like science fair projects. Now that we have computers, making the displays is a cinch. I type in what they tell me, and they glue the printed words and other eye-catching materials on a posterboard. Sure, sometimes my kids need more adult assistance with their projects than teachers think (or hope?) they will.* However, this annual ritual gives my children a chance to plan their own experiments and make their own observations at home where there is no "right answer". (Not that I'm above telling them if their conclusions are nonsensical....) Proper experiments can be quite difficult to plan and carry out. Just making sure that observations are accurate requires some knowledge about what can be measured and how best to do it. I think that my children's beginning efforts in doing science experiments will give them a greater knowledge of both the power and limits of scientific experiments.
* No worries, my children won't be getting awards they don't deserve due to my assistance. Their projects and posterboards aren't really competitive, based on what I've seen at their school in the past. And surely a project where Mom helped is better than a project that didn't get done at all because of a child's fears and indecision. This is just elementary school! Still, I look forward to when they can go solo on these projects.