There are items that are so important
Camera, viewfinder, value scale. They do help with composing a scene and the value scale can help with light and dark color relationships.
Sketch book. Making value studies and other preparatory drawings is a good idea and can make all the difference. If time makes this impractical then get on with painting, but you take your chances.
Comfort items: Hats, umbrellas, sunscreen, raincoats, spare jersey, food, water, folding chair, cell phone.
Basic safety issues need to consider. Where you paint may be unsafe due to wildlife (whether on four or two legs). Environmental issues, extreme weather and any other issues that may also present themselves. Take precautions. Tell friends where you are going to paint and make sure you have a backup plan. Take a painting buddy along.
The above lists seem like a big task, but most of it is getting the items that you already have organised into a system. After a few times out you will have this system working. For example I have a panel carrier and pochade box in my vehicle so it is possible to stop and paint. At least grabbing the missing items and dashing off to a nearby paint spot need not take more than a half hour.
Tips about painting
Use small painting panels: ideal for me is the standard 20cm x 25cm MDF or masonite panel. It is small enough to allow me to complete the painting in about 20 to 30 minutes.
If the painting goes well the panel looks great framed in an oversized moulding. A grand statement piece! Bigger panels can work too, but you may spend too long on it and miss other opportunities with other scenes.
Pre-prime and tone the panels. This is a big help. I double prime the panels in gesso or oil based primer for artists (not regular hardware primer).
Then tone the board with a wash of diluted oil paint. A warm tone like raw sienna, burnt sienna or even red make good landscape panels. Sometimes a cooler neutral is good such as ultramarine.
Have a few options. My panel carrier can hold 12 panels so I have a few options on hand.
As mentioned above it helps to have an idea about what it is you want to achieve. Do you want rolling hills? farmlands? sheep or cattle? people on the beach? Wien you find a good place to set up you will need to work quickly.
I set up my easel and squeeze out my basic paints onto the palette. Line up my brushes and make other arrangements getting my lot ready. I then get out the sketch book and make a couple quick value studies to establish the darks and lights and maybe the mid-tone areas
Check composition in a viewfinder and take a few quick photographs. I will then draw in the main shapes and plot important points on the panel. These points will correspond with the points established in my composition. For instance where a road starts and ends. Where the focal point is and where the horizon line is. Then I get stuck in with painting straight away. No dawdling to second guess myself.
Remember: ESTABLISH THE HORIZON LINE FIRST
Keep direct light off your palette and panel. It may be necessary to paint under an umbrella or shade of a tree for example. If not possible then at least make sure your palette and panel are out of the direct light so that you do not overcompensate with dark values. Keep colors bright and they will look good indoors.
Shade trees are heroes to a lowly, overheated plein-air painter. (Brenda Behr)