Hi BlueRobin,
IMO, I believe Collegiate Activities refer to your ECs in university. In US, I understand college refers to university. If you have won awards, competition, leadership position, etc. in university, you should definitely write about them. Post-Collegiate Activities refer to your ECs after graduation from university. This will be what you participated in outside work. Volunteering, if you are involved, is definitely one area you can discuss. Hobbies will be things which you do on your own and not through any organisation. I guess you can share hobbies which are related or aligned to your aspirations/values, or unique hobbies, if you have any, so you would stand out as an interesting/fun candidate to have in class.
Anybody else care to share your views on this?
All best!