I've adapted an idea from Chan Dissanayake, for a very portable little sketch kit. He made his own box, but I had a little box that originally had a very cheap kid's art kit in it and it's perfect for my needs.
Inside I carry gouache tube paints, a few brushes, a couple of small palettes, some tissues, and a pad of watercolour paper (I'm using Daler-Rowney Bockingford, 300 gsm, 10 x 7 inches (254 mm x 178 mm - between A5 and A4).
I also carry a roll of masking tape (the heavy paper kind from the hardware store) and a couple of very small plastic bottles.
Here it is:
I tape a sheet of paper to the lid and paint on it.
I also carry a bird identification pamphlet as I come across lots of birds when I'm out sketching.
The gouache paints can be used like watercolours but can also be used as an opaque medium, so it's very forgiving - also, quick drying - so it's ideal for quick plein air sketches. A sketch this size can be done in around half an hour.