What inputs does an iPad have? What possible ways of reading input does an iPad have? Right now I can think of:


Touch screen (multitouch I believe?) 
Camera 
Brightness adjuster (How does that work? Could it be used for
anything other than the brightness adjustment?)
Orientation detector 
The standard input buttons


Is there anything I'm missing?
Besides the obvious USB / Lightning connector and the WiFi / Cellular / Bluetooth technology, the iPad has:


Volume up / down switch
Silent / Screen rotation lock
Microphone
Built-in speaker
The gyroscope
The accelerometer
The ambient light sensor
Micro-SIM card try
Facetime / iSight camera
Multi-Touch display


Bear in mind that these are for the latest version.