Depends on the time of year, I'm sure. The beach isn't Disney property. By law, the beach belongs to everyone; it's one of those delightful quirks of Hawaii. That said, it's still inside the Ko Olina country club, so it doesn't tend to get too busy. At least, it wasn't April 2012 when we were there. I went out to lay on the beach at 8AM one morning and had the entire lagoon to myself. It was
glorious.
Are the pool and rec areas crowded?
Again, this is going to be highly dependent on the time of year, but it wasn't in late April. It was busy for sure, but I wouldn't have called it crowded. The pool is Aulani's best feature, though, so expect there to be people and you won't be disappointed.
Is there good snorkeling in the lagoon, or do you have to go somewhere else to see good coral?
We did some "poor man's snorkeling" just using some goggles, and it was great. We saw all sorts of fish in the lagoon. There's a time in the morning when they go out and feed the fish, so that time is the best, but there are beautiful fish there pretty much throughout the day. You might get a better experience elsewhere, though, since the water was a tad murky.
Aulani also has a private salt-water snorkeling tank. If you want the best possible experience, that's the way to go. You can then use that same snorkeling equipment out in the lagoon later. If you don't want to pay for the snorkeling in the tank, there is a viewing area where you can see the fish and stay dry.
Is the lagoon Man made or natural?
Man-made, but you won't care. Trust me.
Can you go boating / sailing / kayaking outside the lagoon? Do they rent boats at the resort or anywhere on the island? (I love to sail on the ocean)
Yes, you can.
Does disney arrange trips to other areas of the island that are of interest? Or do you have to arrange your own snorkel / kayak / sightseeing trips?
Yes, Disney has a staffed island excursions team that can assist you. Just know that you'll be paying Disney prices. You'll save money by booking on your own. A little research can go a long way.
Do you need to rent a car to get around? Or is there plenty to do on site?
You don't need a car, but you'll find it very useful. Oahu is a small island, and it's very easy to tour the whole island in about a day by car. It's also helpful to drive the five minutes to the shopping center to pick up provisions for the room, if you don't want to pay Disney food prices.
