Now for medium size portable painting kits that have their own case or can fit in a backpack or messenger bag. They come with a variety of supplies. You can add some of your own like I did here.
This entire kit zips up into its own case. It even has its own wrist strap. See it in use here with a description of contents. It's still small enough to use comfortably in the car.
Additional materials shown: small plastic mixing palette, 4"x6" watercolor sketch block and tiny Altoids mini kit. I can use the Altoids kit for quick field sketches in the small sketchbook. For larger field studies, I squeeze out fresh paint on the plastic palette and use my 9x12" Kilimanjaro Paintbook, a 10x14" watercolor block or 1/4 sheet 200# Saunders Waterford paper.