I bought some support for my camera and one of the features is a mattebox which holds two 4x4 filters. I started with a Schneider Optics .6 (two stops) to try and get f4 on my 50mm. I may get the one stop .3 next and stack the glass.
a polarizing filter cuts down on reflections. A graduated filter is darker at the top than it is at the bottom, so the sky and the ground can both be properly exposed at the same time.
I think what I want is the polarizing filter if it helps with the sky, but I don't know if it'll just make the sky blue, or if it will allow clouds to be seen too.