Sounds like it is all coming together for you.You will have a lovely day and if you have the two times to work to,namely Sheep World and Ransoms you can use the van to drive around at will.
Your driver should know the sights to visit, or drive through, but here are a few suggestions.You probably wont have time to do them all but can have them up your sleeve, time permitting.

Puhoi
Just off the main road on the way North.A small, historic village, settled by Bohemians.Claim to fame a pub.Further up the the valley,worth a drive for the scenery,is the Puhoi Cheese Factory and Cafe.Lovely country setting,you can look through glass to the cheese factory but being a Sunday may not be working.Handy for a "comfort stop" .
Warkworth
A country town worth a drive through.Has a nice wharf by the river.
Parry Bush Reserve
Also home to the Warkworth Museum.The Museum would be too time consuming to visit, but in the grounds are some HUGE Kauri trees.Worth a photo stop.
Dragon Fly Gallery
On the road to Matakana.Cafe and Gallery worth a look .Also has art work.A modern garden setting.
Matakana
Delightful country town.Has a farmers market, but only on a Saturday morning.Also home to Morris and James ceramics.
Sandspit
Further on from Matakana.Brick Bay Drive takes you to the top of the hill for stunning views over Kawau Bay.A bit further out ,so that one would depend on how you are runnning for time.

Then do Sheep World and on to the winery.
On the way back,
Wenderholm .
A public reserve with a lovely beach amidst big and ancient pohutakawa trees which should all be in bloom for you. Should also see sheep grazing.One spot on the dunes has a giant picture frame where you can take a framed pic with the sea and islands as a backdrop.
Waiwera
Worth a drive through as its just by the highway.Thermal pool resort,in small bay.When the tide is out you can actually dig a hole in the sand and hot water will come to the surface.
Orewa
On the left side as you are returning to Auckland there is a lookout above the beach.
Also coming back into the city just before you cross the bridge there is a great view across the harbour to the city from the motorway.You wont be able to stop, but a good spot to have cameras ready.
That should be a pretty full day,and be able to be done in your time frame, but should you arrive back earler than expected get the driver to carry on down the motorway for a short while and take you for a drive through Cornwall Park and up to the top of "One tree Hill",(now known as "No Tree Hill").From there its just a short run along the motoray and back to the City,or you could head down to waterfront and come back along Tamaki Drive.
David.