Here is a really good example of what you could do with one NXT Retail Set and no other parts or sensors needed. A Rubik’s Cube solver!!! It uses just the retail kit. It seems like a simple design but would be a complex program. The following is some specks on it:

• Scanning the cube: 1 minute
• Calculating a solution: 20 - 40 seconds
• Executing the moves: 1 - 5 minutes. Average 4.5 minutes (60 faceturns)
• Average total time: 6 minutes

Here is his website and a video of it in action.

http://tiltedtwister.com/index.html

(Edit) The Maker has added the LDD File and Program files on his site, so head on over there and build one for yourself.