I'll posit that there is no "alternative" to classic rock. Or, possibly, ALL music that isn't classic rock, is an alternative to classic rock.
I mean, it's kind of a weird question, if you think about it. Jazz is absolutely an alternative to classic rock. So is opera. And so is Tuvan throat singing (which I personally believe everyone capable of hearing, should hear at least once in their lives:
Tuvan Throat Singing - YouTube )
I think I get stuck on the word "alternative." The implied question, by using that word is: What music is not classic rock? The answer is: everything else.