Drive a DC motor rated for higher than 5V
The simplest way that I know is for you to use an H-bridge. It will take PWM to control the speeds, which is easily output by the Arduino. The only downside is that you will need two supply lines: One for the Arduino and one for the bridge. They can run off the same supply, you would just have to use a LM316 or similar to get the voltages you need.

TI has a common h-brigde: http://www.mouser.com/ProductDetail/Texas-Instruments/SN754410NE/?qs=AMJt07B76uuZ4Fb3eRjJ6A%3D%3D&amp;gclid=CKDTkIj-mb4CFYtDMgodyjUAJA