Red Rocks, around from Island Bay, a bit of a walk, but you can see seals there.
The Kapti Coast has lots of nice beaches, QEII park can be great. On the way back stop at Southwolds Car museum.
If you have a car, try to get to Staglands, North of Upper Hutt, it has lots of birds, kuni kuni (native pigs) and stuff to climb over. There are Keas there, take a museli bar and feed them.
If you can, try and get to Kapti Island. It's a bird sanctuary, run by the Department for Conservation, the birds are scarily tame.
The botanic gardens are nice, especially as you can take the cable car from the centre of town. The have a nice little kid's playground and an observatory.
Macra Beach, past Karori, is a stony, windswept beach with lots of interesting things to pick up. Peter Jackson filmed much of his first film, Bad Taste, there.
If you're feeling adventurous, fly-by-wire is at the base of the Wainuimata hill. The little one can chase sheep while daddy flys.
Finally, go see my mum, she'll feed you scones and keep the small one occupied for a while.