What to Include in a Plein Air Kit

We've often been asked what an artist should to bring when going plein air painting. Not that easy to say, as we are all so different, however, I can say what I bring. (This is for acrylic painting.)
First, you need to control your selection of paint. Depending upon how far I expect to walk with my kit, I bring either a bare minimum palette of colours, or a slightly expanded set.
Bare minimum set
Hansa Yellow Medium
Napthol Red Light
Quinacridone Magenta
Burnt Umber
Indanthrene Blue
Titanium White

Expanded set
Hansa Yellow Medium
Napthol Red Light
Quinacridone Magenta
Burnt Umber
Indanthrene Blue
Phthalo Blue (green shade)
Phthalo Green (yellow shade)
Titanium White
Polymer medium
With either of these two paint kits, you can mix almost any colour you come across.

Two brushes: Catalyst by Princeton Polytip bristle 1" & 1/2" Brights (great brushes for plein air, if you like stiff, easy to control brushes)
Palette: Non-stick baking tray
Field easel (I have my own custom-made easel) or pochade box with tripod
Yoghurt tub for cleaning brushes
Rags for cleaning brushes
Viewfinder
Small sketchbook
1 soft pencil
Eraser
Grey-scale markers: black, medium and light non-bleed markers
1 small spritzer bottle for keeping the paint on the palette open
Clamp-on umbrella to shade painting board and palette
3-4, 12x16 (or whichever size you like to work with) painting boards for the day, already primed orange or black
Stool (If you paint sitting down)
Backpack large enough to carry all of the above except the boards, which I carry in a board holder
Water for painting, if you will not be near a water source
Camera to shoot scene and capture progress of your painting

Non-Painting Items
Sun hat
Sunscreen
Insect repellant
Drinking water
Snacks
Sweater or similar in case it turns cool
Rain gear if there's chance of rain
Pliers, for opening stuck paint tubes
Good walking shoes
Cell phone for emergencies

If you are going deep off the regular track, these other items are good to have as well
Compass and/or GPS
Food & more water
Whistle for signalling
Basic first aid kit
Walking pole
Map of area
Space blanket