How to keep my mom happy when she's here visiting?

I agree with others who point out it if probably no longer possible, even if you were willing and able, especially if she has short-term memory issues -- she won't remember that you've been entertaining her. See if she will look at a magazine or thick book with lots of mostly pictures (maybe baby animals or something she doesn't have to "work" at to understand). My 98-yr old aunt with advanced dementia was entertained for quite a while by the same Cat Fancier magazine over the course of many months. I also purchased several DVDs of her favorite, happy old musicals and that's been working well (at first it didn't!)

Just poke around and see what works. What works this week may not next. Maybe she can do a sorting task? Or folding towels and pairing socks (even if you have to buy a whole bunch of socks for her to pair). I have my mom cut rags from old, clean t-shirts with a fabric scissors. Would she listen to music with headphones? Or audiobook that wasn't a complex story? Or, maybe nothing works and you will need to practice being ok with this. Do not feel guilty over this, it cannot be helped and there's no perfect solution. May you have in your heart no matter what.

With your mother’s many health conditions it’s likely that “happy” is over for her. It’s no one’s fault, it’s just that life gets hard with old age. No matter how hard you try to be entertaining the distractions may be brief. My dad best liked simple conversation, though we weren’t dealing with dementia. My mother just liked our presence, it’s not always about doing something

You don't give us much to go on, but I will say this; your mom probably isn't entertained 24/7 when you are not visiting, so I certainly wouldn't feel obligated to do so now. Just spend time with her. Take her out for lunch or supper, shopping or just a walk around her neighborhood. I'm sure she's enjoying just having you there. Just enjoy your time with her, and quit putting undue pressure on yourself to have to do more.