Well, I'm rather biased as I play piano, so it's near and dear. Plus the thought of those long, spider-y hands moving over the keyboard is...enticing. X) But I could definitely see a string instrument as well; violin or cello. Imagine Snape furiously bowing out a Bach piece on a cello fitted snugly between his legs...

::spends several moments daydreaming::

There's arguably some proof in canon that Snape can sing (the incident with Draco), although for me there's something much more intimate and expositional about singing, so even if Snape can sing, I somehow doubt that he would.

I agree with you about the Princes. I interpret Snape's nickname to mean that he is proud of being a Prince, which makes me think that it has some significance...but not necessarily importance. It's perhaps significant because the Princes used to be a well-known pureblood family, but no longer. Eileen might have been disinherited like Andromeda Black and that's why the Snapes had no money (or so we're meant to infer, I think), but I kind of fancy the idea that the Princes were like the Malfoys many generations ago and then with every generation, lost more and more money and status until by Eileen's generation, they were like the Weasleys. Except much smaller in number, heh.

I'm babbling, sorry. Snape does that to me. X)