I recently picked up a 59 cent package of blank index cards for doodling. After I was doodling on a piece of paper that I was also taking notes on during a meeting at work, I somehow ended up with a paper of doodles from a coworker and another coworker took my paper (with doodles & notes) and passed it to the doodle giving coworker. I protested and my paper was ripped in half and I got my notes back. I realized that something as simple as index cards would help alleviate this cross usage. So that's where I am.
Although I love the Pigma micron pens, I do sometimes wish there were more colors available. Then I discovered the Stabilo 88 fineline markers, which come in 30 colors. My mantra for pens is usually the thinner the tip the better, so the Stabilo 88's at .4mm are a compromise for the color variety in this manner, but they're still pretty nice. I approve! Also, considering you can get a set of 25 for under $15, they're also relatively cheap.
Anyway, that was a really long way of saying that I've been doodling on index cards with Stabilo fineline markers! Here's the first few cards in which I repeatedly explore the circle...: