You may be painting in acrylic or oils, and you might like canvas, canvas board or even sketchbooks of bound canvas (or canvas-textured paper). There are also a variety of hard-surface painting boards, as well as watercolor paper already mounted to board. These are handy in the field but are heavier to carry. A canvas pad or canvas-textured paper can be fun to sketch on, as it offers interesting textural effects.
You can really see the texture of the canvas pad in this pencil and watercolor detail of my sketch at Watkins Mill. The texture helps capture the feeling of the rainy day.
Many watercolorists are experimenting with Yupo “paper,” which is really a thin plastic prepared for painting. It doesn’t buckle under a wet wash, but because it’s completely nonabsorbent, your paint (or other medium like ink or graphite) sits on top. It can make interesting puddles and can be lifted and manipulated easily. It’s fun, though challenging.