Nancy,
This is one of my FAVORITE questions! What you do that first afternoon/evening really sets the mood for the entire vacation. And like you, I don't want to rush off to a park after we get there.
I would suggest that you start by easing into your vacation with some resort pool time, if you have time. Really reminds everyone that it is a vacation! Since you get in at 4, maybe only for an hour or so, but still a great thing to do.
Next, plan a special and festive dinner. I would say you could go with a dinner show like the
Spirit of Aloha Show, or the
Hoop-Dee-Doo Musical Review. But if you want to keep it a bit more affordable and still watch the Magic Kingdom fireworks, I would recommend a reservation at
'Ohana. Get a table in the main room where you have a great view of the Magic Kingdom. If you time it right you can watch the fireworks from there. If not, you could always go outside to the Polynesian Resort beach to watch, where they pipe in the soundtrack from the park!
Have a Magical Vacation!