Switch off Wii U GamPad - Sync Button Mode

The easiest way without pulling the battery out of Pad or walk away with it is done this way:

1) Press Sync Button on Wii U twice. (GamePad shows message that it is already synced)
2) Press On Off Button on GamePad. Only GamePad switches off. Wii U keeps running.
3) Now you can switch on your Wii Mote and press B to go back to main screen.
-> Now you are running your Wii U without the fancy Gamepad until the next reboot.

PS: The GamPad is completely switched...

Switch off Wii U GamPad - Sync Button Mode