For me, the most important item of painting outdoors is my easel. My personal favorite is the workhorse called the Take It Easel. The TIE is a Gloucester style easel that opens into a large tripod with crossbars to stabilize it. At about 8 lbs. it is easily slung over the shoulder (with rifle strap) and carried wherever I need it. I posted a short video regarding its setup and breakdown a couple of years ago on YouTube. Here is a link to that video. This easel is so well made it's hard to beat. I have painted up to 30" x 40" canvas on it, and down to an 8" x 10".
I made the paintbox that I sit on the cross bars out of scrap wood I had, and added purchased hardware such as the piano hinge, handle and latches. I also scavenged the canvas holder of off a French easel and attached it to the box. It measures approximately 20" x 20" x 4" thick when closed. It lays completely flat when open which gives me lots of room to sit stuff on the inner lid.
The paint I use is made by a small company in New York state called RGH Artsists' Oil Paints. Great company, great service, and a wonderful product. I normally buy in jars and tube my own.
I just bought a new backpack. The Kelty Redwing 50 is a dream. Lots of pockets and a main compartment that unzips completely for easy packing of extra clothing, shoes, etc. I love it so far.
Equipment Detail of Backpack Contents:
Candle (for waxing canvas holding rod on easel)
Baggie with Pens, Pencils and Small Clips
Baggie with Nitrile Gloves
Extra Battery for Camera
Some of these items are "just in case" items, but those don't weigh much, so I don't worry about keeping them in my pack. You never know what come up when you're out in the woods by yourself.
The small canvases in my pack are there as a "just in case" also, as I usually don't paint under 16" x 20" in the field, and those canvases are held in the holder on the box.
I hope this gives you an idea of what, how and why I pack what I do to go paint in the great outdoors. If you have any questions, please let me know and I'll see if I can answer them for you.