What grade level? That will certainly depend on what types of projects you might choose.

Here are some ideas:

What (substance) cleans pennies best?

Does aspirin keep cut flowers fresher, longer?

What causes fog?

How does a volcano explode?

What is the tensile strength of a human hair?

Is it possible to crush an egg with your hand if the pressure to the egg is applied equally and if not, why?

How accurate are the nutrition charts in fast food restaurants?

Which plant will grow better, one that 'hears' classical music or one that 'hears' rap?

Good luck on your project!