This is the kit I use when I am doing plein air sketches, it contains:

• my lovely W&N pocket watercolour box,
• some extra brushes (old cosmetic brushes tend to be small but good quality)
• a couple of pencil stubs and a pencil holder, eraser, pencil-sharpener,
• a purse-sized kleenex pack,
• a natural sponge,
• and postcard-sized watercolour paper