Above: This is a spread in my handbound journal using Strathmore 500 Series Mixed Media Paper which is now available in sheets. (The journal is approx. 7 x 9-3/8 inches.) I worked with a couple almost dried out Faber-Castell Pitt Artist's Calligraphy Pens. I used left over Schmincke gouache from my palette and just kept with mixing purples and magentas. Click on the image to view an enlargement.
I'm not a big fan of doodling—for myself. Typically I'm ready to move on to another page. But sometimes late at night, after I have sketched and after I've written something, it seems to me that doodling is a way to keep playing with paint and not start a new page (though there would be nothing wrong with that, but again typically I'm not in the mood to go to bed leaving pages unfinished).
Doodling is a great way to try out the marks a tool makes, whether that tool is a dried out marker or a particular paint brush. Across this spread I can find eight experiments, wait nine. Things I just wanted to try on this night.
Take some time to experiment. Doodling will make it seem less serious, but the results you get will be just as valid as information so that you can thoughtfully plan your next project.