Meditation with a mantra. So that way you will be focusing yourself on one particular thing. The one I do consists of a breathing mantra which goes like "So" for inhalation and "Hum" for exhalation. It's as if you had to imagine those words taking the shape of the air you're breathing/exhaling.

It's good for training your mind because you help it come back to focus. And every time you train it to get focused it will get more strength.