We went the day before we checked out. It was fantastic. If you book the last appointment of the day, they let you go 90-120 minutes before your appointment to enjoy the amenities. The pools, the ice bath, sauna and steam room.
OK, I'm back to add more details.
We arrived at Aulani on November 6th. When we arrived, our room wasn't ready (we got there around 1 pm and were still waiting for our friend to arrive, her plane didn't land until 3 hours after ours did). So, we wandered around, ate, and then figured out where the spa was (in the W tower, we were in the E tower).
We asked for available appointments for the three off us, around the same time, for November 7 and November 8 and the only availability was 5:30 p.m. (2 appointments) and 6 p.m. (1 appointment) on November 8. We had the Signature Lomi Lomi and I barely remember the sticks, only the hot stones. The receptionist let us know that since we had the last appointments of the day, we could arrive 90 minutes ahead of our appointment time and since the 3rd person was at 6 p.m., she could arrive with us, so 120 minutes prior to her appointment time.
After walking us all around and pointing out where things were, they left us to do our thing.
We had time to enjoy the 5 mini-pools (varying temperatures), the ice bath a few times (me, I couldn't get in all the way), went to the steam room for a few minutes ( we can last maybe 5-10 minutes) and then went to the sauna (same thing, we can last maybe 5-10 minutes). Rinsed in the mini-shower outside to rinse off the chlorine off our swimsuits, before going into the steam room, then sauna, then went to the women's showers, where we had enough time to wash our hair and ourselves (so no need for a shower later that night), dry our hair and then we got the 15-minute "warning" that our appointment was in 15 minutes (well, those whose appointments were at 5:30 p.m.) so we headed to the relaxation room with massage chairs til we were called.
*note: they do tell you during the tour that wherever we are, they will find out to let us know that we are 15 minutes from our appointment time*