The biggest argument skeptics have against global warming is, they say, that carbon dioxide and water vapor somehow coordinate together to keep the greenhouse effect a constant. This, as far as I know, has never been proven so. So I would propose a simple experiment to prove this or not. Fill a vessel with water vapor + carbon dioxide and measure the greenhouse effect of several different levels of concentrations of each. This would be a simple experiment as far as I know it hasn’t been done.