My altered book project is something I do for relaxation, but you still need an idea. There really is no idea with Zentangle. You draw these "strings" to divide the paper into sections to get started, then just doodle. They have a website at zentangle.com where you can learn more.
I have a bad wrist so I'm not so sure how it will hold up to holding the pen for so long but nobody says you have to finish it in a day, right! I'm looking forward to trying some of the patterns in the book. Maybe a little sampler. Maybe a very slow sampler!