An automatic door opener. With a pir sensor and a motor to open the door.
A device to walk up straight, with a detector high on someone his/her back that makes a sound when not walking up right.
A walking stick that has a TV remote inside.
A computer with skype that turns on and swings in front of a person laying on bed at a voice command.
Sensors in the floor that detect a person or wheelchair and determines the direction, and automatically opens the bathroom or toilet door.
A device that automatically cooks something or makes coffee with a single button.
An automatic water system for plants or for a green wall with moss.
You can have a look at the Playground section : http://playground.arduino.cc/
Click on the left on "Interfacing with hardware" and "User code library". Explore those sections, there should be perhaps hundred usable ideas for you.