fiddilng around with bright colors and the airbrush tool for no reason beyond the tactile pleasure of using MS paint
Honestly, MS paint feels better than just about every other art program I've ever used.
It allows so little subtlety, you're either putting down a tiny thread of color or a huge smear of color, barely a middle ground.
Then there's the lack of antialiasing, and how when you save as a JPG a blanket of ugly compression instantly bleeds all your colors together, solidifying your drawing in a way.
It's just wonderful, I highly recommend it. Pick up the pre-win7 version somehow if you can, the new tools don't have the same feeling at all.