Road trip in Oman (Riyadh)

Dear all
I am planning a camping road trip in Oman in a few weeks time and I wondered if anyone had been there and could give me some advice on places to visit.
My wife and children are flying to Muscat from the UK and we intend to hire a 4X4 and head off into the blue yonder.
I have heard that camping is allowed just about everywhere, so we want to plan a really good trip taking in as many of the sights as is reasonable.
Any info you might have from where to pick up a 4X4 at a reasonable price to places of interest to visit would be gratefully received.
Many thanks in advance
Tom.