I haven’t painted outdoors much, but the few times that I have, I’ve sketched scenes quickly in ink (black Sharpie fine point pen) and watercolor pencil (Prismacolor).
The hardest things to draw when sketching outdoors are things that move. Humans, birds, dogs, cats…these are all difficult to sketch quickly, so you have to approximate proportions and try to keep things in memory for as long as possible.
Many professional wildlife artists have a habit of sketching things quickly in the field. Some go on hikes with a sketchbook and draw anything of interest that comes into view. Even if their objects of interest scamper away before they’re drawn to satisfaction, these artists claim that sketching increases their understanding of the world around them. After sketching stuff regularly for a few years, I agree: sketching is a powerful observational tool.
Today was the Hermann Park Kite Festival, so I walked over, found a comfy spot with a good view, and sketched the following scene in ink and watercolor pencil:
I’ve created similar plein air sketches with ink and watercolor pencil in the past. Here’s one from May, 2013. It’s a sketch of Torrey Pines State Reserve as seen from North Beach, in La Jolla, CA:
Finally, this is a sketch of the Pacific Ocean as seen from Torrey Pines Gliderport: