Park yourselves in the sand dunes at Wells Beach for digging, paddling and picnicking, and then walk through the pinewoods behind.
Go on a trip to see the seals from Morston - check trips and timetables here: or google 'Blakeney Point Seal Trips for one of the other 3 or 4 companies that run ferries - reserve tickets and then turn up at Morston Quay in plenty of time.
Get Fish and Chips from Mary Jane's chip shop in Cromer - it's the best one. Or sit on Wells Quay with chips from French's chip shop.
Take an inflatable dinghy in the shallow creek at Blakeney at low tide.
The Shire Horse Centre for looking at horses and ponies, and a cart ride, Amazonia Zoo, if raining the Steam Engine Museum at Thursford is indoors and has original steam driven carousels and steam driven dance hall organs that they play in turn - also good play area and nice picnicing garden. If desparate there is an indoor soft play place in Wells at the quay end of the beach road. Also miniature train down to the beach.
Go to the duck pond at Salthouse or Glandford Ford (Glandford between Wiveton and Letheringsett) and feed ducks, and pop into the little shell museum.
N Norfolk is a v outdoors bucket and spade sort of place.
Visit Sheringham - you can go from Cromer to Sheringham on the train.