Hi Rae,
What a lovely space!
Looking at the plants that grow there, I guess it is pretty shaded and moist. That sounds like a great place for ferns and mosses indeed.
I have heard, but never tried (because I dont need to because I have ferns and mosses growing all over the place) that "painting" walls with buttermilk or yoghurt, will make mosses grow on them. You have a lot of walls that could use the mosses to make them more interesting looking. I would start with a strong visual shape, like a circle or a block on the concrete wall. While you work on the house, see what happens. Later you can decide whether you like the edges neat and maintain them, or just let it spread wild.
If the place is as moist as it seems on the pictures, all kind of things, probably ferns as well, will start to grow spontaneously in the moss a year later. If not, you might need to help it getting started. In that case, go for plants that love acidic soil.
Hortensias do well in a moist shady place if you like to bring in a big dot of colour. Azaleas are beautiful if the soil is acidic. Don't forget to plant some white flowers, they light up in darkness. Woodland plants is what you should probably be looking for.
If the patio is actually a lot sunnier than it seems to me, the concrete walls will store a lot of heat that they will release during the night. That offers great opportunities for all kinds of fruit. A well maintained blackberry or raspberries on the trellis will provide some fresh fruit as well.
If I were you, I would change little things, one at a time. It is a great place, you don't want to ruin it and than find out you don't like gardening all that much.
Good luck, have fun and let us know how it works out!