One of my coworkers is heavily interested in and quite adept at developing curriculum using the iRobot Create platform. It's a wonderful platform with two wheels and multiple sensors including encoders, IR sensor, touch sensors on the front and sides, and the ability to detect an edge in front or when wheels dip. It has bluetooth capability with an optional module, Arduino-esque capability with an optional module, and I believe there are wifi modules (a la Raspberry Pi) which are compatible with the iRobot Create.
My coworker and I ran a three day workshop on robotics with the goal of building and programming robots which could play soccer via remote control. My coworker has done a huge amount of work on setting up an HTML -> Python -> iRobot platform so webpages can be written to control the robot. He is also very interested in using them for remote teleoperated applications using readily available technologies like Skype to handle the remote viewing operation.
For this workshop, I spent a couple weeks working on a way to install a kicker on the iRobot. I had a 12V solenoid, and thought that I could use it, driven through the iRobot's battery, to "kick" a ball. I designed the kicker in SolidWorks to have a lever which pulls an arm forward when the solenoid is pulled in. This is the "kick". I wanted to do this when the solenoid is energized, because otherwise the robot battery will drain all the time except when you are kicking. 
The setup for the lever is a good example of a third 
class lever. The pivot is at the end, with the effort in the middle and the load at the top. The load is the force required to kick the ball. Third class levers amplify the distance traveled and speed of the load relative to the effort, since the load is positioned further away from the pivot than the effort. The trade-off is that the effort must be larger than the load, since the torque exerted by the effort must be greater than the torque created by the load, but the distance from the pivot to the effort is less than the distance from the pivot to the load. 
I 3D printed the kicker and laser cut out a base platform, two side panels to mount the solenoid, the lever, and the pivot point. I used a spring around the ram between the solenoid and the lever to provide a restoring force to move the kicker back into the robot when the solenoid electromagnet was released. I designed a simple and compact electronics board to drive the solenoid based on a digital signal from the iRobot. Four wires from the electronics board to the robot connect ground, battery power for the solenoid, +5V for a relay circuit, and a TTL signal to drive the relay. I used a voltage regulator to step down the iRobot battery voltage to 12V for the solenoid. I 3D printed a nice little box to hold all the electronics.
Electronics Board Files
The input and output pins for the iRobot Create can be accessed using a series of bytes sent (via bluetooth in this case) over a serial port to the iRobot microprocessor. I played around with the bytes. Check out the work in the two links below; the first is the manufacturers document which describes the available commands for hacking the iRobot. The second is my notes to show what I was able to implement. iRobot Create Interface
Practice Document
