This is like saying, "What to pack for hiking in Nepal or Nebraska." The answer is always, it depends. The things you pack are always a compromise between completeness and compactness. Pack what you need for the circumstances, and leave everything else at home.
Here is what I suggest that my students bring for plein air painting, not too far from the car, in Portland, Oregon:
-A SUN HAT, clothing for the weather, sun shirt, sunscreen, insect repellent (which can sometimes be left in the car.) Weather conditions may include heat, cold, light rain or wind, and may change throughout the day. Pack the clothing you need to be comfortable. Give careful thought to your SHOES. You will be standing a lot, and standing pretty still. Your feet need to be warm (or cool) and comfortable. GLASSES if you wear them. You will not want to wear sunglasses, as they will alter the apparent colors of your paint.
-DRINKING WATER, SACK LUNCH OR SNACK.
-YOUR PAINTING ESSENTIALS, packed in the smallest, lightest package you can figure out. A rolling box works well. A backpack covers terrain more flexibly, but will carry less.
-A PLEIN AIR EASEL, light easel, tabletop easel, or plastic garbage bag to lay your painting on.
-It's also nice to have a chair, umbrella, and camera, but these are optional.
A complete oil supply list, including colors, which you may want to pare down a bit for plein air.