What inputs does an iPad have?
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.