I got several books for my birthday and decided to read 2 of them at once: The Creative License by Danny Gregory and Keys to Drawing with Imagination by Bert Dodson. Both books have exercises in them so I'm alternating between the two.
These are from the Bert Dodson book: Make 6 random doodles then noodle the interiors any way you want.
All these were done with a micron pen. They took forever but I love doodling and so I didn't mind. My hand sure is tired though.