Practising the guitar, piano, vocal or even drums can be great for improving concentration. Of course, if you can't play then music lessons with a friendly competent music teacher will help you speed up the process. Focusing becomes a habit for your mind and that focus then translates into other areas of your life like school, work and other hobbies.
Here is a great article from the University of Alberta that explains the different areas of the mind that improve with regular music lessons or just casual jamming at home.
What are you waiting for? Dust off that old instrument that has been collecting dust over the years. Contact a music teacher or music school that is close to you and move your mind into the fast lane