Thanks for the feedback. We are pretty sold on the hike but were curious if it's worth booking through Disney. Funny but I got off the phone with the booking agent and he talked me out of it! Said it's easy to just rent a car and do it ourselves. Only benefit of booking through Disney is you get transportation that's it. Hopeful4fl do you have a recommendation for parking?
Please put Manoa falls within your smartphone and it will lead you right to it. PLEASE watch out for speed signs once you turn into the neighborhoods. The roads are windy and I saw 20 mph almost all the way.
Now to the parking part. There is a restaurant there called the Treetop that has an upper level parking with a sign they will tow. From talking to people, they do tow, but these people were the ones collecting the money for the lower parking lot ($5), so they had a vested interest. My advise is to pay the $5 and take all stress out of it.
IN CASE the lot is full, my advise would be to back track to the neighborhood (maybe half a mile or less) and then walk to the trail.
If you do find parking at the lower paid lot, the road to the trail is right next to it. There is a paved road that goes up about 300 yards and then the actual trail starts beyond the gates.
This is a hike I think you would like. As I said, we actually had plans at Waikiki, and since you are now driving, make sure you do that too. Here is what I think would be an optimal plan, but this is what "I" would do and not saying this would work for you:
Leave Aulani at 9:15 to arrive by 10'ish. THIS IS TO BEAT RUSH HOUR. I would do this on a weekday to take local people out of the trail.
Spend exploring Manoa for 2-3 hours. This would put you around 1'ish
Go to Waikiki. If you plan to go to Duke's, MAKE RESERVATIONS. If you plan to go hit the beach directly, find a good joint to eat. I would recommend Tommy Bahama restaurant if you don't want to do Duke's. There are dozens of other places you can pick from.
Alternatively, you can do Diamond head too. If too much for a day, just take in the sights of Waikiki.
Regardless of what you end up doing, I am sure you will have fun. Be safe on the trail and enjoy your time.