I've had a similar situation happen to a friend of my recently, who's A-Levels were brilliant and has been rejected several times by Oxbridge (all D1s and A*s). They were all post-interview, however. I find it unusual that someone would be rejected pre-interview with grades like that... the only thing I can suggest is that they request feedback on exactly where they went wrong. I know of someone that got rejected pre-interview for Cambs but she had AACC.
Regarding extra-curriculars, other than subject related activity, I don't think they care. In fact, my Director of Studies has openly asked me (begged me) not to do drama or rowing - or any other sport that requires a lot of time and application. Oxbridge didn't get their reputation by having students with lots of extra curricular activities; it's their long history of academic excellence and teaching that give them their reputation.
If you're on track to get AAA it's really not going to make a difference whether or not you're a grade 8 on the piano if they have to pick between you and someone else that is on track for the required A*AA but don't happen to have a Grade 8. If you have experience relevant to your course i.e applying for History: worked in a museum, that could go in your favour but people are very naive and misinformed to think either a) having all these extra curricular's means a candidate will be at a much better advantage than those without them or b) not having a long list of extra curricular's will be a real blow to someone's application - not everyone has the same opportunities as others do, playing an instrument being one of them (can be very expensive).
Don't pick up loads of ECs just for the sake of it to impress interviewers would be my advice. Do activities relevant to your subject and that will enrich your knowledge and further your practical experience of the subject.
That's my advice anyway. I'm sure there's typo's everywhere in this but ah well