A couple wires, a diode, a 5v to 9v usb adapter, and you are in business. The reason I did this was to make a fish feeder work for at least a week. I was going away and didn't want anyone to be burden with fish feeding.
Usb cable for NXT brick (included in Mindstorm kit)
5v to 9v usb adapter
A 2 amp diode ( thank you Lan01)
soldering iron and solder
wire (small size from radio shack)
Phillips screw driver
wire cutters and needle nose pliers
Step 1: Opening NXT
Take out batteries and the four small screws in each corner (Not now but in a future step you will need to pay attention to the little black button in the low middle of the picture). Turn over brick and pull on the white face plate. Might be a little tough but comes straight out.
The board is attached to the screen by 3 items. 1.) Two screws one on each side of the screen. 2.) The 10 solder spots. If you just pull up from that spot it should come right out but be careful because the side with the speaker has (3.) 2 wires that do not separate. So fold the screen over on its face. You can put something soft down so as if not to scratch it (as you can see in the third picture I did not. Nothing was scratched but better safe than sorry).