If your 6th grader is studying the weather, how about this idea from the Boy Scout merit badge book:
Make one of the following instruments: wind vane, anemometer, rain gauge, hygrometer. The library...
You can do a tornado in a bottle. I had fun with that. get two clear bottles (those 1 liter soda bottles work great) and colored liquid. You will also need a connecting piece. you can buy them at a...
I will recommend a project I did with my daughter when she was in grade 5: the assay of Vitamin C in citrus fruit.
This is an excellent project because it is of potential use, even for...