I started Zentangles with my middle school this year and they loved it. Once they knew how to do them you can have them for extra activities when they finish. I have students who come down to the art room to get a square to work on one in homeroom. I used tagboard which is cheaper than watercolor (the official paper). I also made a big poster like they have on the zentangle website where they can masking tape up their finished zentangles. I talked to a lady who taught this at an art workshop and she uses a big zentangle (10 x 10) to teach with to see patterns before they do the mini ones.
There is a zentangle website that publishes new doodles once a month. I run a copy of them for some students who like to copy instead of making their own patterns.