We were at Aulani this past June and the only thing that seemed like you'd need a reservation was the character breakfast at Makahiki if you wanted a certain time. Amu Amu may need something if you want a table right at sunset, but nothing seemed that busy.
Monkey Pod across the street takes reservations as well, but we made ours same day.
If you know you want something in particular or are going during a really busy time it won't hurt to book now, but we didn't have any issues making things same day.
By far the best Disney resort ever!!! Enjoy!