I also like using just black ink. Lately, I've been using this Platinum Preppy extra fine for my doodling. I like how different kinds of pens and tools can sometimes give different art.
What do you see?
I usually don't have a plan when I put my pen to the paper. A lot of the time, I just draw what comes to me and sometimes it starts to form into other "bigger" shapes/objects like this one.