Activities for my mom with middle dementia and vision problems?

Singing.
Old show tunes, church hymns if she's into that, Frank Sinatra or someone from her generation who she liked, songs from when we were all kids...
i.e. Old MacDonald had a farm eieio. On that farm there was a ___ eieio.
i.e. You are my sunshine, my only sunshine, you make me happy when skies are gray....
You get the idea.
I think it helps prompt their memory too. When you start the song, then they pipe in with words. My grandma came up with all the phrases to songs where I only knew the first part. I think it made her feel really good inside too because she remembered or knew portions of songs I didn't.

I'd say "Let's play our name that tune game". It wasn't really name that tune the way I described it above, but name that tune would be another idea too.

Music was a lifesaver for my mother. She had dementia and macular degeneration, and that was the only time she seemed truly happy.

My dad loved, loved to make bird feeders. Get a pinecone. Put peanut butter on it and roll it in birdseed. I would attach jute twine to the top and hang it. I put everything in a box top, so it didn't spread everywhere. I have the best pictures of him smiling while he made these.

Listening to music, short audio stories, singing?

A 2nd idea is giving her a bunch of washcloths or papers to fold. It doesn't fall in the category of a "fun thing to do together" and "mind-engaging" in the same way as my last suggestion. But my aunt used to have my grandma do that & it made her feel like she was helping my aunt with something that needed to get done. (Plus broke up boredom after vision probs hampered reading/writing).

My song-game suggestion was more fun though! (Just suggesting it for your mom brought back happy memories of my gram's last years!)

Grab some cheap dried flower and make a few arrangements either for gifts at Xmas or for around the house. My MIL loves playing w flowers and making arrangements.

Has anyone discovered the magic of YouTube renovation videos? Not the silly stuff with the so called “influencers”, but professionals who do beautiful restoration, conservation and refinishing work. They are often beautifully produced with lovely music, visuals and soothing narration. I sometimes show my husband my favorite furniture restorer and even though furniture was never his thing, he finds them mesmerizing and even calming. Commenters on these videos often mention that they have anxiety or sleep disorders and they find the videos to be therapeutic.

If you have a tablet or small laptop you can set her up with a 25 minute video and it may be all she needs.

My MIL would fold napkins for the table and laundry which may or may not have been refolded out of her sight.. It kept her feeling useful. My daughter carved a pumpkin with her watching and asked her what to name "him", how many teeth to give him, smile or frown etc. which made her feel involved and stimulated and engaged. I would ask her to dictate a list of ingredients needed for a recipe since her only interest or hobby had been baking. It gets boring for us be she enjoys repetative activity.

I had a client who lost her ability to read. Her eyesight was fine, but her brain couldn't do it any more. I read stories to her, she especially enjoyed classic fairy tales. We also read a long book, but every time I gave a quick review of the story from the beginning to our present spot.
I gave my Dad a Fiddle Muff that kept his mind focused and his hands warm. It's a knit tube about 18 inches long made out of different weights and textures of yarn in many colors. Attached to the inside and outside were lots of different things like a house key, a dog-shaped keychain, a metal nut and bolt, charms, McDonald happy meal figures, small toy cars, Lego blocks, anything with interesting texture. He liked to feel all the things and would run his fingers over every item. We also played a game with it where I asked if he could find a particular item. Sometimes I had it and he would ask me to find something on the muff. I was amazed that he could recall what was there but didn't know my name.
Many knitting groups make these. It's a great way to use up little bits of leftover yarn.

NLEPHD, regarding vision, I found a really great magnifying glass made by Carson [Staples carry it] as I have problems seeing up close. It works best for reading the newspaper columns and looking at photos.

Speaking of photos, any old photo albums around? I remember my folks had a lot of them and there were photos of people I had no idea who they were. No writing on the back.

I took up doing a family tree, and I could have kicked myself for not doing it sooner so I could get get the information I needed from my parents.