Hi Tomasjoad,
Christmas is a crazy time to visit Exmouth. VERY Hot, very dry, brown and dusty! Won't be any grey nomads up there at that time of year, they all bail around October/November due to heat. Call Scotty of Ningaloo Kiteboarding if you want the inside skinny 0427046376. You'll find him in Nimitz Street as you come into town on the left across from the caravan park.
Problem with aircon in vans etc is they take quite a bit of power to run them. Yardie Homestead ask you not to use them as they generate their own electricity and the added load causes problems for all guests with power outs. You cannot use generators in the national park so Aircon is pointless in there. It might be OK in town though (for aircon), but then you're in town. It may be your only respite from the heat so may still be well worth it.
If the missus is into kiting, great, I'd head to Yardie or the National park on the west coast of the peninsula, if not and she wants to be close to cafe's, food and shops, then she'll be far happier in town. There's a great little water park that will be 'busy as' in town. There is nothing for miles when staying at Yardie, everything is in town. You may or may not even get phone reception depending on your service provider at Yardie.
Personally, I'd go no further than Gero and head south instead. The South coast has even more beautiful beaches than up north, the water is much cooler, you can head to Margies and hit the vineyards and the local attractions. Good surf all along the southern coast and South west corner. No surf in the north at all over Christmas. If heading down south over Chrissy, you WILL NEED to book as it will be packed EVERY where.
WA is hot and windy in Dec/Jan, so going up north, you will need tiny kites as the further north you go, the windier it will get.
Enjoy WA, it rocks in Summer, a kiters paradise, but I'd be avoiding the North personally at that time of year.
DM
PS. If going up north in Summer, you will NEED fly nets! I Sh1t you not! Often times you can go up north and if the flies are bad (if you haven't been up north when they are bad you cannot even comprehend how bad, "bad" can be) no one will have fly nets as they can all be sold out! (Personal experience). Fly nets are more important than sunscreen in the north!