I'd say it's somewhat down to personal taste, but a good guide is to check what's working well in the neighbourhood gardens.

Re sourcing this time of year - You usually find local plant fayres and village fetes can be a good option.

I spent a few minutes wondering what you meant by a 'hard water area', I was imagining a lake with hard surfacing round it, but its just occurred to me you might simply mean the water supply is hard...

How big is the area you want to plant, and what aspect (sun, shade, lots of wind, frost pocket, etc).

Hi Red maple, 'plant and forget' and 'show stopping colour' don't really equate in my book... If you want a fantastic looking garden you are going to have to put some work into it on a regular basis - if only to remove the weeds which will appear.

Perennial geraniums more or less look after themselves once established (which for some can take a year or two) The colours are mostly bright blues and pinks. (Don't confuse these with the bedding "geraniums" which are actually pelargoniums.) Euonymus fortunii varieties of small shrub grow fairly slowly and are evergreen, ma,y brightly variegated and good for winter colour. Winter flowering heathers (ericas, not callunas) will grow into clumps and give bright pink or white flowers in late winter - lovely with yellow crocus, which will also come every year. The heathers are best trimmed lightly with shears after flowering but otherwise look after themselves.

But for all these you will have to do some weeding as dandelions and various cresses will invade whatever you do and need to be got rid of regularly.

I live in the same area. If you really want to plant and forget and yet have beautiful colour, I think you are talking about shrubs. I have ceanothus (Californian lilac) which gives abundant spring colour, hebes, potentilla and cistus give good summer colour. Photinia will provide a beautiful maroon. Euonymus, elaeagnus, ilex, provide yellow and green throughout the year. Plant these and sprinkle a few bulbs of daffodils and many colours of hyacynth and you can sit back and enjoy.