There are quite a few spots around Kaikouria. Quite a bit has already been discussed in previous posts.
There are good sites close to the township, or you could drive either south or north. Good spots in both directions. Although the sea can be rougher to the south.
North of the Clarence there is a very nice strech of long gravel surf beaches while north it is more rocky with smaller beaches.
Maybe on the day you drive up to the sounds it could be worth a stop at the three fords. That is where the main road goes through three fords, quite easy to find. That whole beach is fairly good.