Our nature journals are spiral bound, thick watercolor paper. We have used watercolor paints, colored pencils, ink pens and watercolor pencils in our journals with our favorite being the latter. The watercolor pencils are easy to use and result in fabulous effects with a dab of water on a small paint brush. There is no drying time and perhaps best of all, they came in a very flat metal box exactly the same size as the journals. This pile of three items sits nicely in the back window of my car waiting for inspiration to strike, which definitely happens sporadically.
We have even wet our brushes in a splashing fountain to swirl and darken the colors of the stripes on a lizard's back.
On occasion Marianna and I will treat ourselves to lunch out at a restaurant on the water and share a sandwich and jar of water for our paints.
A favorite trick is to draw the trunk of a tree, getting the shape just right, and then take multiple leaf rubbings to fill in the canopy.