Hi Suzy
it is a difficult thing to generalise about. First place to start is as Karen says, with things that have given pleasure in the past and that may continue to do so, at least before things go too seriously downhill.
Anything that works now may not work next week.
I have a friend whose mother had Alzheimer's and when he was trying to pass his experience to me for my wife, he told me that he found quite accidentally that his mother liked one of his Everly Brothers tapes. So he put it on continuous play throughout the day and that was good for her - and for him too. I found that familiar music could be helpful. The shame was that Jan was a superb pianist, and she lost that very quickly.
We were always great gardeners, Jan and I, and during her final year at home, I completely redesigned the garden closest to the house, to make it more friendly and accessible for her [we are on the side of a valley, the garden is 400 feet long and terraced, so we retreated to the top terrace, perhaps 60 feet long, 60 feet wide]. We were able to walk, talk and enjoy the flower beds, sit in a new arbour, watch the birds and the cats, have picnics in the sun.
I found that TV was the least useful thing. I tried to get Jan interested in watching East Enders regularly, but she was always too intelligent for that, even when ill. An unexpected bad side effect was that I became totally hooked for quite a time, to her frustration.
Talking with her - even when neither of us knows what on earth we are talking about - has been the longest serving activity. This is something quite precious to the patient, and few care home staff ever do this once the person is resident there. That personal contact is invaluable, but time-consuming, and often draining of energy.
Now our best activity is crawling together round a small room.