Watercolour Plein Air Painting Checklist:
Watercolour paintbox or selection of tubes
At least one brush
Pencil and eraser
Four (rustproof) clips or pegs for keeping your paper in place when it's windy
Paper towel or cloth for wiping brushes on
Bottle of water
Cup for washing brushes
Watercolour paper
If necessary, an empty bottle for pouring dirty paint water into, for disposable back home.
Tips: A largish zippered pencil box or toiletry bag is ideal for putting your brushes etc. in.
Retractable brushes take up minimal space.
One of those blocks of watercolour paper where the paper is 'stuck down' is ideal because you don't need to stretch it, but you will need something sharp to separate off a sheet once you're done.
Consider buying a watercolour field set a small box of paints complete with retractable brush; some even have waterbottles.
Plein Air Painting Checklists: Acrylics
Oils
Pastels
Other Essentials