Apart from the graffitti in the surrounding area and the dystopian tram stop underneath, NDK is quite nice. It may not look like much on the inside, but it's an architectural marvel on the inside. Much more tangled than the Sofia University (and I love it). My father knew the architect and he sad the man was a genius. Also, do show her the National theater and the nearby former King's palace which is now the National art gallery.
As for out of Sofia... well, there's so much stuff to see, one wouldn't know where to begin. But I would definitely recommend Koprivshtitsa - http://en.wikipedia.org/wiki/Koprivshtitsa - it's an architectural and historical preserve and is in the list "100 Tourist Sites in Bulgaria". And it's closer than Plovdiv (which is also a great place to visit, especially the old town)