Plenty of sea kayaking around Auckland...
Not recommending the company below (I have no knowledge of them, but their website was the most informative I just googled) and have never sea kayaked to Rangitoto, but can recommend the island itself as a place to visit!
http://www.nzkayak.co.nz/auckland-nz-tours/rangitoto-is-day-kayak.html
Do you rock climb? If so I'd say head to Mount Eden Quarry if you have a morning or afternoon free. I was last there a few years ago, but I can't imagine it has stopped being a friendly place to climb some nice lines and relax! Also likely to meet folk heading a bit further afield and you may be able to join?
Edited to say that the reason I say 'plenty of sea kayaking' when I've never done it is that there were always lots of them out and about along the north shore coast!