Smartphone-controlled Zumo Robot for Arduino Tank
This Instructable by Annikken shows how to turn a Zumo Robot for Arduino into a smartphone-controlled tank. It uses the Annikken Andee for Android (they have one for iOS too), which is an Arduino shield that is connected between the Arduino and the Zumo.
Here is a video with the tank driving around:
More details are available on the Instructable page.