Of all the different media that can be used to sketch and paint in the field, watercolor paints are the simplest to stick in a pocket or backpack and just go.  This little gem of a plein aire kit is the simplest, smallest, and most complete designed to date.  12 half pans of colors perfect for landscape painting, two fold out mixing palettes, a #2 good quality brush that breaks down and stores in the short handle, a plastic water bottle, and the kit cap holds it all together.  There's even smidgen of a sponge and a pivoting ring for hanging the kit on a backpack or belt.