If you need to add user input without using a button, then a capacitive touch interface might be the answer. The AT42QT1010 SparkFun Capacitive Touch Breakout offers a single capacitive touch button with easy-to-use digital I/O pins.
The AT42QT1010 is a dedicated, single-button capacitive sense chip. The chip handles monitoring a conductive area for touch. As long as a touch (e.g. from a finger) is detected, the AT42QT1010 keeps the output line high. Otherwise, the line is kept low. You just need to provide a power source (1.8V - 5V) and ground for the AT42QT1010 to work. Additionally, a PAD pin is available if you would like to create your own external electrode.