Are the brown thumbs lack of experience or lack of inclination? Because plantings would be a simple and cost effective first step.
Also
- what's your budget?
- which direction does the front of the house and living spaces face? (ie are you going to want to put an outdoor living area at the front, which will mean you'll want privacy, or is the front only ever going to be visual space?

Also, are you looking for immediate effect, or willing to wait a few years for things to grow to height. Is the curb appeal to appeal to buyers, or just generally look pleasant?

You have some beautiful bush out back. Complement that by planting some trees in your garden. Visit a local nursery for ideas about what will be suitable for your space and locality.

I think the most impact would come from a well placed, well sized Acer. You can get them grown fairly tall already, Tamata have them and I'm sure there are other places. I'd get one that grows to no more than 3 metres (although check this against your property as scale is hard to gauge from photo), and has changing colours of leaves/bark. I'd place it towards the left of the property, looking at it. That will give some balance to the windows, provide some privacy but not obscure light. It wouldn't be a cheap option but cheaper than painting the house, and would need almost no aftercare. Then some easy care phormiums in the border underneath the windows. Both phormiums and acers come in a range of colours- limes, bronze, purples, oranges, dark reds etc, so choose what you like. The acers are soft and provide movement and variety, the phormiums sharp and provide solid consistent structure. This echoes the plants that can be seen already behind and at the side of the house and should make the hour 'sit within' its environment instead of bing perched on top of it.

Also, the edge of the driveway- I would dig a border to just beyond the seam of the gate, lay down a weed mat, and put some stones in there. The garden centre has a range of them, in several colours from white through green red greys and blacks.

If you chose reddish (or reddish and lime/orange) tones for your acer/phormiums, I would prob buy a large specimen plant in a pot for the top of the driveway, at the eight hand side of the garage, to link the colours. And maybe paint the front door and have some pot plants there that tone in too, especially in the area near the path that can be seen from the street.

For a cleaner line I'd consider painting the lower part of the decking (the vertical frontage) to tone in with your house.

A nice house that has been modernised. I would plant up the front of the house which will take away your concern about the windows not being centred. Go for taller plants on the left of the front of the house. The new door stands out brightly and it has nothing to complement it so I would take that colour and get some pots or flowers to blend in and place them on the deck. Your house looks tidy and clean and that is most important for street appeal.

I would paint the house a strong colour with white edging. I would put big planter boxes on that deck and fill them with large colourful flowering shrubs ( that means little work) - that adds to privacy. I would definitely paint the front fence and add a camellia, kowhai or some such for the centre of the front lawn..

To answer questions:

The front right corner of the house faces North (in the Southern Hemisphere this means the noon-sun comes from this direction), so the front windows get good morning and midday light, while the sliders and windows on the right (driveway) side of the house get the evening sun.

Even though the main living areas face the street, any outdoor entertaining we do is generally at the rear of the property - as that is more private, green and has elevated views of the mountains.

I've thought about painting the house and garage black, with white trim; but, my husband really doesn't like that idea - so we might go with blue instead. I like the look of white trim set against a dark colour; but, since everything looks okay at the moment, I think we will hold off on that job for a year or two.

We just finished an expensive upgrade, so our budget is almost non-existent at the moment. We have no intention of moving ever again, so we are interested in playing the long-game when it comes to enhancing the property - we just want to add/change things a little at a time.

We are not into gardening, so any plants we get need to be low maintenance.

Thanks to everyone for their advice.

Cheers, Linda