I do love sketchbooks and notebooks of all sorts whether they are handmade or commercially produced. I tend to gravitate to the medium size for both art journaling and travel journaling, somewhere around 5x9 inches.
My go-to notebook for this summer was the smallest yet. It was a beautiful, Rhodia 2"x3". The pages were coated and smooth, perfect for pen and colored pencil. I used it strictly for sketching wild flowers around Oregon in the early spring and summer. But I usually have 3-4 notebooks active at all times. Some are better for watercolor washes with heavier weight paper, and others are for everyday writing and tracking details about daily events, like the Moleskine below.
The above two sketches were done with a slightly new technique. First, I sketch the scene with a line drawing. Then I filled a watercolor paintbrush with diluted India Ink (because it's what I had) and painted in the values. Straight from the brush was a medium gray. To get it lighter I rubbed it out with my finger or used water. To made a darker shade, I waited until it dried and layers more black ink. The minimal use of color was added last. This process is a great study in finding value, light to dark. (The notebook used was my everyday moleskine, 3.5x5 in, lined light weight paper, pen was Pilot Precise .5, w/c Cotman travel set)
This shows the layering process starting with a simple line drawing, then adding watercolor and colored pencils. I chose to use more realistic colors, but another option would have been to use arbitrary colors, a more playful, whimsical approach.