Having gear ready to go with everything necessary for a great plein air experience is the first step to getting spontaneous... and getting outside!
Take larger brushes with a palette with large wells to accommodate. Large field bag carries boards (facing each other), easel, stool, jacket, etc.
It took a workshop experience with Nancy Taylor Stonington (www.nancystonington.com) to convince me plein air painting was not only fun, but could actually result in fresher, more spontaneous paintings. Even if a painting is not completed in the field, starting outdoors with ambient light, the ability to move around the subject area, and the exhilaration from being out there sets the tone for finishing the piece later in the studio. Put together a field kit that is ready to go whenever you are in the mood. You certainly could go with less, or haul around even more stuff - but this has worked very well for me: The mainstay is my roller cart with a lid - becomes table top for sketch book, field palette, bamboo brush holder (with just a FEW brushes!), water spray bottle, and collapsible water container. Small case for drawing pencils, Exacto knife (for cutting masking templates, sharpening pencils, holding bears at bay, etc.), kneaded eraser, masking tape, Masque pen mastic and pickup, main tubes of paint in a ziplock, and camera (or phone camera!). Stretched paper on gator board is lightweight and makes a perfect rigid 'board' for a small field watercolor easel - better yet, take two and face them together for protection in the field bag. A field bag is more versatile than a portfolio as you can stuff it with jacket, hat, collapsible stool, and field easel. (This type of easel is capable of almost horizontal positions necessary for watercolor painting.) Add a waterbottle of fresh water (for drinking as well as pouring into water container(s)), roll of paper towels, and maybe an umbrella if you have a personal Sherpa to hold it for you :-)
Most stuff will fit into this crate. Palette should be closed and traveling in the flat position!
Nancy recommends some pre-painting scouting expeditions. Sleuthing around ahead of time will help you find paint-worthy sites, maybe determine the best times of the day, and possibly get permission from landowners if that's an issue. The advantage is to be in place and set up when the light is right, and also to be able to return on a different day to finish if you like.
There is potential for too much sun, wind, too many bugs, rain, and other outdoor mini-hazards. As Nancy says, they just come with the territory. Wear a sun shirt and hat, umbrellas can be helpful for too much sun/heat, use repellent if you have to, just 'woman up'! Changing light and weather are also challenges, which can be minimized by taking camera shots for later reference. Many painters will finish work begun in the field back at the studio.
Start with a reasonable sketch, whether thumbnail or larger - get your editing and problem solving done before committing to the watercolor paper. Another tip from Nancy: do the sketch, solve the problems and get it on your WC paper one day - go back at the same time with similar weather to actually paint it. Accompanied by several photos, the sketch(es) will provide emotional support when the light changes or worse and you have to quickly pack it in. The next thought is NOT to go miniature - if you are accustomed to working on 1/2 sheets or larger - DO IT!!
A palette with a rubber seal keeps paints moist and INSIDE for no-mess travel.
Facing two stretched sheets attached to gator board gives rigid support to the bag and protects the paper surfaces.
Is it worth the effort?
That'll be up to you, but I have noticed a definite freshness in pieces I have painted on site - whether related to the speed of painting, or racing with changing light - I tend to think less and paint more! Give it a try :-)