You don't have to make reservations at Ama Ama or Makahiki, but I would recommend it.
Without a reservation, you're at the mercy of any crowd that happens to be at the resort on any given day. The restaurants at the resort are expensive, even by Hawaii standards (a nice dinner for two at Ama Ama can set you back $160 without alcohol. Basic breakfast for four will run $100.).
You can plan activities once you're at the resort, but remember that you will be paying for the "Disney Experience". That isn't a bad thing, but you'll pay more for the outstanding service.
You didn't say if you were renting a car or if you were staying in a villa with a kitchen.
Renting a car:
DVC Members aren't charged for parking, so if you rent a car and want to see the island (I would kill for a slice of chocolate haupia pie from Ted's right now), you don't have to have the car back by 5:00 to avoid the overnight parking fee. If you have a GPS, bring it (or
Avis has them for rent). In Hawaii, it is illegal to operate your phone while driving, so getting directions from your phone while driving will get you in trouble with the law.
If you have a car and want to go to Pearl Harbor, Ala Moana, Waikiki (yuck), or any one of a million another gorgeous places, remember that Honolulu traffic is busy. Rush hour starts at 5:30 or 6:00 and lasts until 9:00 and then starts up again at 3:00 to 3:30 until 6:00. Traffic is busy every day of the week so plan extra time to get around and to find a parking place.
Villa
If you're in a villa and you want to prepare your own meals, the closest market is the ABC Store across the street from the resort. It's certainly not a mega-market, but they have some of the basics and they're expensive. We found it better to rent a car and drive to Target and Foodland.
Activities
What I miss most is sitting by the pool listening to the everything around me; children laughing, background music, and the water. Floating down the lazy river was one of my favorite passtimes; my son and I had a blast on the water slides.
Most importantly, have fun and enjoy yourself.