Hi Hendrik, what a great project you are working on.
Good suggestions already from other posters. You will of course be taking care to avoid poisonous plants or those whose leaves could cause skin or eye irritation.
If it was me, I would take this a step further, and design a productive kitchen garden with a wide range of salad and herbs, vegetables and (non-prickly) fruit. Since most of these will not have much flower interest you would add structure and colour with the addition of herbaceous plants. Hebes and lavenders are good because they have a long flowering season if you keep them pruned. Lavender is also great as a sensory plant.
In a residential setting some people will enjoy supervised gardening, which is where a kitchen garden is particularly ideal. It is gardening with purpose, and so many people with dementia are longing for purposive activities.
My other observations about gardens for people with dementia are based on personal experience, and fall in to two main themes.
1) REDUCED VISUAL PERCEPTION
Many people with dementia lose their ability to recognise colour as others would normally see it. It is like a sort of 'fade to grey'. The brain apparently filters out the information that it does not find essential. Flowers that are grown for display should therefore be brightly coloured, or white, contrasting with foliage.
My MIL used to love her garden, and visiting gardens open to the public, but she doesn't seem interested in plants any more. I personally think they have all gone grey in her perception. If you could only see plants in shades of grey you would conclude that they were dead or dormant, wouldn't you?
In the last couple of years that MIL lived at home on her own she struggled with the amount of work that she thought her garden required. Actually her son looked after it all for her, but she was always fretting about this or that needing doing. Strangely, some of the things she talked about were not relevant and didn't need doing. It was as if she was recalling some past season rather than being able to actually 'see' what was right in front of her. She continued to fret about the garden when she went into her first CH. She avoided supervised gardening, saying surely they had people to do that! Then in the CH where she is now she was back to fretting about needing to tidy up the planted areas and pots, so she didn't like being taken out into the courtyard gardens.
A couple of years further on, and she now does seem to like the gardens but not really for their plants. She just likes sitting on a bench in the sun. BTW, plenty of comfortable benches is a must in your garden. There are always some CH residents who go outside for a smoke or a walk on a regular basis. They tend to have their favourite seats and this can mean there is nowhere for anyone else to sit down because the regulars do not welcome other people invading their space. Benches also need shade from the sun and shelter from the wind. A bench surrounded by a timber arbour can have climbing plants around it, which can be enjoyed by people who are not strong enough to walk far.
2) PHYSICAL ACCESS TO PLANTED AREAS
My mum was a very keen and skilled amateur gardener. She designed her current garden when my parents moved into their new-build house 13 years ago, when he was 84 and she was 78. It was mainly planted with shrubs and trees to be low maintenance, but so well designed as to have all year round colour interest. She was a proper gardener - unlike me - I haven't got that level of planting knowledge.
I have in the last couple of years had to prune and restructure much of her garden due to the age of the shrubs and trees. My mum's original shrub plantings can be seen across a lawn from where she sits in the living room, although some are still recovering from the necessary heavy pruning! I also wanted to give my mum a close up garden experience from her wheelchair, so I extended the paving around the perimeter of the house and replanted the beds close to the house adjacent to the paving.
This has given me the opportunity to remove all the prickly shrubs that might hurt her, and to plant some new things. I have put in some plants that are at eye level or below as she goes past in her wheelchair, including some annuals because of their vibrant flower colours. I have focused more on spring and summer flowering, but with some autumn and winter flowering heathers, and early spring bulbs, in case she does go outside in colder weather.
My greatest success this year was a wildflower bed, which I think would go very well in your sensory garden. Any sunny area would be suitable, including a raised bed. I used a well-drained slope. A wildflower area is a great use of a sunny slope. The bees and butterflies love it and the flowers have been blooming continuously from May onwards (with seeds being sown in April).