We got there early... the kids got leis, we walked around the resort, and there was quite a long line of people checking in for their reservations at Ohana. After dinner we hung around for a while, took some pictures of the kids on the beach. Honestly, it was the most enjoyable evening of our trip. I wouldn't try to rush there and back.
Also, you can take the small boat instead of the monorail. As long as there isn't a crowd too big to fit on one boat it's just a few minute wait and ride across the lake.