Isn't the toy fair in itself a really fun thing to do??
In all seriousness, I'd recommend visiting the city's webpage for events/fun things to do. From a quick googlesearch, I found it's URL (ain't I a nice guy?
):
http://www.nuernberg.de/english/
If I were you, I'd also stock up on German pastries... YUM!
EDIT: Don't worry about the messed-up spelling of the URL... City names are often different in languages other than that of the foreign country...