Cable car (from Lambton Quay) up to the lookout and the park up there. Waterfront is great on a day like today, get some ice cream. Drive around the bays, or walk to oriental bay. Wander up cuba st. Wander up lambton quay, not at lunch time it's full of people.
If she is adventurous, averagely fit, has a good 3-4 hours to spare and has access to transport (train and taxi will work) I recommend a trip out to Porirua and do the Adrenalin Forest, especially on a day like to day.
After I was born, I didn't talk to my parents for two years.
Wander down Cuba St and have a coffee at Fidels or Midnight Espresso. Head up to the Wind Turbine or Mt Vic to enjoy the view (on a nice day). Hit Cuba St again at night for some cool bars or Courtney Place for the drunken debauchery depending on your taste\age. Walk along Oriental Parade and grab a Gelato
Weta Cave in Miramar - Weta Digitals homage to everything they do Carter Observatory in Kelburn - Stars, The Universe etc. Sit in the Parliament Public Gallery during question time. Fairly interesting at times. Wellington City and Sea Museum Catch the ferry over to Eastbourne and have an ice cream on the wharf.