Breathing is one option, although it doesn't strictly need to be the focus. The goal is to focus your -entire- mind upon your focal point, leaving no mental space for anything like boredom.

Honestly I'm not sure how applicable mindfulness meditation is to different mental types. Some people seem genuinely incapable of focusing their entire mind on one thing; I'm on the extreme end in the other direction, and am incapable of conscious focus on -more- than one thing at a time, to the point where I can be in extraordinary pain and -simply not notice-, because something else is more interesting. To some extent mindfulness meditation probably just reinforces a habit in me which, while generally desirable, is over-exercised in my case.

Breathing is good for beginners.

I would recommend against focusing on an object outside of your body at the beginning for people in the LW cluster.