If mountain bikes, then then Kelling and Salthouse Heath, inland between Cley and Kelling - good tracks etc. Also the pinewoods at Wells - excellent for walking, cycling and tree-climbing in. Wells is one of the best beaches in the area - park down by the beach along the beach road, walk along the beach and back through the pinewoods, or through the woods and back via beach - fantastic! Wells also has a lovely quay, with a good chip shop - French's
Get a ferry to see the seals on Blakeney point. Ferries can be booked from Blakeney quay, but actually leave from Morston. or book at Morston in the pub! Or book online beforehand - times are subject to tides- google Bishops Ferries, or Temples Seal Trips.
Cromer has a small interesting museum, which shows ancient fishing life and also the excellent fossilized finds (a wooly mammoth) at west runton, the next beach along, where you can go fossil hunting.
There is a birdwatching centre at Cley, with telescopes and a camera onto the marshes.
Sheringham is sweet, a different version of Cromer, Holt is a v chi chi little Georgian Market town with smart shopping - and a surprisingly good toy dept above the newsagent, Starlings. I think there is also a pottery cafe in Holt.
Hope for good weather. Unless you are v hardy and have proper waterproofs, N Norfolk is not the best in wet weather - though Splash, the Steam Engine Museum at Thursford, and trips to attractions further away will keep you busy.