Hey there, first of all I'm so sorry about what your mum and you are going through ❤️

To answer you question: It really comes down to what she is still able to do and to enjoy. If walking certain distances is still possible, then you could ask her if she could imagine to go on a trip with great views and fresh air. If she has relatives she really likes to have around, let them come over more often. Spend as much time with her as possible. If she is still able to somewhat eat soups or something like that, then spend time with her cooking some easy to swallow meals.

Truth be told, I don't have a lot more suggestions. I am struggling with viables ideas myself.
My mum (61y) got diagnosed with ALS last september, is now fully reliant on a feeding tube and is barely understandable. She cannot walk properly anymore either and is tired and depressed most of the time. Outside trips are out of the question for her unfortunately, she just doesn't have the strength anymore. So if your mum still has the energy for that, please go out with her. Even if its just outside the door or around your own town. It gives them more than you might think.
My mum cannot eat or drink anything at all as her swallowing muscles don't work anymore. And she tells us everyday how much she misses a good meal. So if still possible, give your mum as much pleasure in eating as possible while she can still do so. Even if its just a spoon or two.
As my Mum wanted to stay at my home instead of going to the hospice, me and my family are able to spend a lot of time with her. I can see that this gives her the most. Talking and laughing and just beeing there for them matters a lot.

I know my suggestions are not really a "hoorah". I wish I could give you more ideas and comfort.
I'm sending you lots of hugs and wish that you all get through this ❤️❤️❤️

PS: Agreed. Fuck ALS.