Hobbies and Unusual ECs

So I have been playing guitar for around 4 years now. I am privately taught and haven’t been able to start a band or anything like that. Could I still include this as an EC when applying for college?

Same with mountaineering, as I climbed the six 14,000ft peaks of the Front Range in three weeks when I was 16. I did it with my father and not a school group, so how would I count that? If I can’t include that at all

Yes, these count. No reason they wouldn’t, as many students have music lessons and/or hobbies that they pursue outside school. Just list them as activities on your application.

It is fine to include activities outside of school.