Hi, when you register them for Aunty's online, you can choose the free activities you want. We were there 7 nights and got 3 free activities. Then a couple days after registering, I got an email with the dates and times of the free activities we got into. I didn't feel the need to change the days or times of the events, but have read that others have called to switch if the free activity overlapped with an excursion. You can also drop in for play. We were there in July and never had an issue with Aunty's being full for drop in, but I have read others having an issue. We would just pop in and see if they had room for our boys and they always did.
We are not buffet people, so we never tried the buffet. If you plan on doing that, I would get a reservation. Seems to fill up fast for the character dining. We made reservations for Ama Ama for breakfast two mornings and for dinner twice, once for us and once for my parents (both had our anniversary dinners there). When I made our dinner reservations, it was less than 180 days and only fairly early times were available (5:00, 5:30pm). We didn't mind. For Monkeypod across the street, we made a reservation the same day that we dined and never had an issue. I just hopped on the website and it looks like you can book a reservation up to early March as of now.