Many times I enjoy working with just a pen, without watercolor. Most of my sketchbook sketches start out that way - just a pen drawing. Because of time constraints I will usually add color later. And sometimes I will leave the sketch as is.
I actually started my art career with the goal of drawing Pasadena with pen in my sketchbook. Gradually I started adding color accents and pale color washes, basically tricking myself into learning how to paint. I found it much easier to add color to an interesting drawing than to face a faint pencil drawing on a sheet of watercolor paper with a loaded brush. The pen sketch gave me confidence to try anything. Consequently, it has become my favorite way to start a painting and as I said, sometimes its how I finish it too.
I usually don't use a pencil either. I start with a pen right away. If I make an errant line, I just correct it with another in the right place. I find that the mistake lines add life and energy to the sketch, and it keeps me interested in seeing what's going to happen next. The times when I block out the sketch with a pencil first, the drawing ends up looking forced and fussy.
This is a scene of downtown Los Angeles from a little pocket park just north of the city.