We did Rangitoto in an afternoon, caught the 12 ferry over and the 5pm one back, its a nice hike to the summit but if you've got kids or aren't too fit it would probably be better to leave it. The devonport ferry would be fun for the ride, you could have a coffee in devonport and the catch it back again.
I agree about Cornwall park (if its a nice day!), I have many fond childhood memories of picknicks, walks and the playground (which probably by now has had most of the fun safety'd out of it by osh).
Another fun thing to do with kids is the high ropes course at Woodhill forrest. http://www.treeadventures.co.nz/
The out of towners usually mock it, but I think they're just jealous that there's so much to do. :lol: