Singing is often good- especially if their language isn't so great- or recorded music. Books are such personal taste and it depends how long they focus for, but old rhymes probably, yes.
A friend bought her mum a photo book with lots of old photos that she had printed and also a wildlife book with lots of photos in it. Her mum loved them.
JJ, my aunt has alzheimers I visit her every Saturday afternoon. I just talk to her about every day stuff, you know family, friends and things like that. I listen to what she say's and go with the flow.
Photos are a good idea from trt. Even if your friends don't remember the faces, you can talk about the people and chat about any memories you have of them and stories you've heard.
Indulge their questions, however repetitive and however childlike. Listen, and help. Just be a friendly face. Great suggestions so fa with reading and photographs.