I strolled around in a local nature area, trying out what I think of as a sort of bare-essentials sketching kit:
It consists of the following:
1. Small sketchbook, about 10 x 15 cm. I like working small anyway, but such a small sketchbook has other advantages. It is relatively easy to sketch standing up, so I don't have to lug a folding chair around.
Furthermore, I have long been uncomfortable with sketching in public, where a whole crowd soon starts to hang around, looking over one's shoulder and making remarks (I swear if I ever again have to hear the question "Why don't you just use a camera?" I'll probably commit murder). With such a small sketchbook one feels more secretive! People walked right by me, apparently not much interested in what I was scribbling.
Last but not least, making such small sketches absolves one from the pressure of feeling that one has to produce a big masterpiece. This makes it more likely that I'll actually get around to sketching something instead of walking around trying to find "suitable subject matter" - with a small sketch, the entire world is suddenly replete with an embarrassment of riches for the quick sketch aficionado.
2. Gel pen. A roller ball will also work, or anything else that easily delivers its ink. I enjoy sketching with a ballpoint, but one needs to put some pressure on it to make a mark, which is difficult when standing with a somewhat floppy sketchbook in hand. With more liquid ink, the ink will easily flow onto the paper. Also, it prevents my perpetual sin of fussiness: there is no way to make very light, tentative lines, so it forces me to be more bold, decisive and expressive.
3. Watercolor pencils: primary colors, for obvious reasons, to which I added a green and a mid brown. I have a more complete set, but I find it difficult to juggle all the colors. With this minimal set I can achieve a reasonably broad range of color and tone.
4. Water brush for wetting the pencil marks. One can of course use a water brush with watercolor pans as well, but I find it a bit difficult to work that way.
5. Craft knife for sharpening pencils.
And that's it. Perhaps somewhat limiting, but also very easy to carry around in a satchel, and can be whipped out on short notice for a quick five minute sketch of whatever catches my eye. I find that in this way I actually get more done than when I carry around more complete and sophisticated equipment.
Some of today's mini sketches: