I have bought Zen Brush and I really enjoy it.

Enjoy performing zen art on your iPad or iPhone.
Zen Brush is an app that allows you to easily enjoy the feeling of using an ink brush to write or to paint. It allows anyone to easily perform fluent strokes while not compromising on the fascinating texture of a real ink brush. Create works that radiate the right atmosphere by choosing the best background template from our large collection. You can even post your works easily on twitter.

So far what I do is more like doodle with a brush but it’s fascinating. It’s mindfulness, full focus on what you are doing, and work as a tool for meditation too. Since it’s digital it’s easy to erase and start over again. I practice with my non-dominant hand too, it’s good to use both hands.