A big part of doodling is adding highlights after the fact, or putting a thin white line around something to help it separate from the background. Upon a time I used to use a White Out pen, however those proved difficult to control, and could easily drop a big glop when all I wanted was a squiggle.
Now, I use either a thin, precision brush with my handy little tube of Windsor & Newton Designer's Gouache for jobs with a lot of painterly bits to work through. Adding a bit of water can thin the white for a more transparent, wash type of effect, or you can just pull some dabs from the cap of the paint tube for bolder, crisper white pops, like the accents on eyes or where your light in the drawing is hitting objects in the drawing the most, especially anything plastic, metallic, wet, or crustacean.
For more general, edge and thin line type accents, I use Uni-ball Signo Pigment Ink White, either the broad or the UM153 from Mitsubishi in Japan, or in my case, via Amazon by the lot. Kudos to my pal Adam Cable for introducing me to these, another game changer.