Hi there and welcome to TP.
As Suzanna1969 says, it is frustrating when your loved one loses interest in hobbies and you feel responsible for providing them with stimulation.
There are two aspects to this IMO.
1. Your own feelings: guilt, frustration, boredom and sadness.
"Couldn't I somehow do more to prevent Dad from withdrawing from the world? The more his world shrinks, the more ours does too."
2. Your dad's wants and needs: mobility, mental stimulation, retaining fine motor skills, avoiding depression, avoiding anxiety, suitable pace of life, maintaining confidence and self esteem, doing things he enjoys, an environment that is focused on his needs not yours, predictable routine with no threats or challenges.
You can see by looking at things from those perspectives that there are incompatibilities between your dad's needs and wants, and the needs and wants of you and your OH. You don't want (or need) to live your lives at the pace of an 80 year old.
Your Dad reacts to negative experiences by choosing to avoid them. He fell in the garden so to avoid it ever happening again he won't go there again. He finds it difficult to read print and to use technology. It is upsetting and embarrassing, so he prefers not to try. He can't concentrate for very long, so gets bored with things like models, card games and jigsaws that he used to enjoy. The very things that once made these interesting to him are now the things he struggles with: mental focus, planning and problem solving, fine motor co-ordination, depth perception and other visual skills.
I think you have to keep trying, but scale down what you offer to a very basic level so that your dad doesn't see 'new' activities as being big insurmountable hurdles that are best avoided. He sounds a lot like my mum was in the early stages of her dementia. She had insight into her diminished capabilities, and was always very risk averse.
Have a look at how you can make his access to the garden safer. Does he need to lean on things, or to sit down suddenly? Has his vision diminished so that he can't anticipate hazards or deal with steps or slopes? Does he need better sunglasses and a hat so that he isn't squinting against glare? Would he accept a turn around the garden before bedtime when the garden is cooler? If he goes out regularly for a fag, use this as a opportunity to prolong his outside time by bringing out some drinks and maybe a snack to enjoy in the shade. I can't cope with too much sun and heat, but I can do 10 minutes before I need to come in and lie down, so perhaps your dad could stay out a little bit longer if he had company and a cool drink?
For indoor activities, ideas might be to help you prepare food, fold laundry, water a potted plant, or choose a postcard to send to a friend. He will probably find this quite tiring. You cannot underestimate the amount of effort that is required to focus on a task when your brain is damaged. Sometimes you are thinking he is 'bored' when actually be has just had enough for now. They are not the same thing.
He can be persuaded to do more than he thinks he is capable of. Just keep it simple and don't show disappointment if he gives up, or refuses. This is what a paid carer would do. They probably wouldn't be as disappointed or discouraged as you are; they would just try again later with something else. I am not saying that you are not a good carer BTW, it's just that as the child you want to please your parent so much that you can feel a failure if things you used to do seem no longer to work. It's like having the rug pulled from under your feet.
Does he enjoy telling stories? Now is the time to capture those stories. My mum told me so many things that I had never heard before. She started to retell the old familiar anecdotes in more factual ways, instead of the way I had always heard them before - with her cast as either heroine or victim! Now that she can barely talk at all I treasure the times we had when she could still communicate well.
I'm sure that's more than enough suggestions for now.
Katrine x