Just got back from a week in Vermont. I did some plein air sketching with Tombow brush markers. These are a great medium for quick, gestural sketching either in monochrome or full color. I started out doing monochromes in shades of sepia or grey, then I broke down and went back to the art store and got the full range of colors. These are perfect for sketching quickly and not having to drag around a lot of equipment and supplies. From the sketches and a few reference photos, you can easily create full scale paintings back in the studio.
Vermont Sketch Book: Marker Drawings