When sketching with a pen I prefer a smooth surface paper. For these sketches I used the hot pressed 140 paper Arches Aquarelle Carnet de Voyage, 6 x 10 inches in size.
My sketching kit consists of my old favorite Rotring Artpen with its watersoluble ink cartridges. I quite often dilute the ink with a Holbein Waterbrush pen. The plastic barrel of the waterbrush pen is filled with water and you squeeze the barrel to release the water. The synthetic hair brush tip allows quite a variety of washes, strokes and effects.
I recently started using Derwent Inktense Blocks which are watersoluble ink blocks that can be used dry or diluted with the waterbrush to create colour patterns or washes. They are very easy to carry around, and the colour effects can be very light or quite intense, as desired.