More neutral:
* Doodle
* Scribble
* Sketch (if it's more drawing-focused)
* Jot (if it's more writing-focused)
* Freehand
* Non-representational art
* Automatic writing (for writing)
* Mind mapping (for drawing and writing)
More evocative:
* Let your hand wander
* Let your thoughts flow onto the page
* Unleash your inner child
* Explore your subconscious
* Give in to the creative impulse
More negative:
* Fiddle
* Mindlessly doodle
* Waste time
* Daydream
* Lack focus
The best phrase to use will depend on the context and the specific tone you want to convey.