you can and should sign up for both in advance, i believe at 90 days out.
go to aunty's beach house's webpage (
https://resorts.disney.go.com/aulani-hawaii-resort/activities/auntys-beach-house/), look for the registration form under Registration (right side of page under Aunty's photo), fill out it, select three out of the four free activities. in a few days (or hours, sometimes) they will email you back to let you know which free activities your child is assigned to. it depends on the length of stay. we were there for 5 nights and got 2; someone who was there for 8 nights and got 3.
good luck! highly highly recommend signing up in advance. nobody wants to wake up early in the morning to wait in line on their vacation!