The beautiful thing about sketching is it can be done just about everywhere. I have several books stashed in different places ready to use whenever I have some time on my hands: in the car, in my work bag, in my purse, and at home. While photography definitely requires the ability to "see," drawing really forces you to notice every detail. If you don't see it, you can't draw it. (Well, you could, but that's a whole other kind of art.)
If you are a sketcher and you want to be a part of something pretty cool, check out The Sketchbook Project at
http://www.sketchbookproject.com/. Basically, you fill out a sketchbook that becomes part of a kind of sketchbook library where people from all over the country can check them out . Pretty cool.