This is my mini version of the Millennium Falcon from the Star Wars films. I built it back around 1999/2000, although years and years ago (around 1984) I built a similar version. There are only three pictures, which are unfortunately very slightly blurred.
There's nothing particularly special in the construction. All the pieces are fairly standard, especially if you have any of the classic space LEGO from the late 1970s to early 1980s (often available on ebay).
There is a wonderful site by Henry Lim that details a fully working harpsichord made out of LEGO. It's the only working LEGO instrument that I'm aware of, although many years ago I saw a LEGO guitar in the pages of Bricks 'n' Pieces and somewhere I have a photo of a LEGO ukulele that I built.
Unfortunately, the string tension required in a piano makes building a working replica difficult, not to mention the number of bricks that would be needed! My plan is to build an octave or as near to an octave as I can get before running out of bricks.
I have already prototyped the mechanism and pictures can be found below. As it can be seen it's fairly rough and ready. The model is based on the mechanism found in a traditional grand piano.
Key to the operation is the Wippen assembly. This allows the hammer to rest back slightly without going all the way back its original position, whilst the key is held. This makes repetition of the note much faster since the pianist doesn't have to completely release the key before striking the note again.