Good old-fashioned pencils. You simply can't beat them. Simple, erasable, versatile. Sharpen them in a contemplative fashion; twiddle or twirl them absent-mindedly; chew on them reflectively - they help the creative processes in more ways than simply being a writing and drawing tool.
We're sure you don't need us to tell you that there's no actual lead in pencils - it's actually a mixture of graphite and clay. Graphite was popularly called lead because it resembled the soft metal. The varying proportions of ingredients give rise to the relative softness or hardness of the pencil 'lead'. We have pencils running from a super-soft and smudgy 10B to a super-hard 10H. Good old HB sits in the middle. B and 2B are nice for general purpose writing and drawing - just that little bit darker and smoother than HB. You can use the filters on the left to find the pencil that suits you best - lead grades are under 'Ink Colour'.