CJ, that sounds like a wonderful treat! I wish there had been one of those spas in the Sacramento area when my mom had her cancer.
Joe, first and foremost it sounds like your mom has a great attitude and outlook on this situation. That does wonders for the healing process. And the fact that she loves to stay as busy as she has always been is going to do a lot to further her successful outcome.
In the last year and a half, both of my parents have had cancer and to meet them, even during the treatment phases, you would never even have known it. They each approached it like, "eh, another bump along life's highway, let's get it fixed so we can get on with things." They each chose radiation as their form of treatment and I know that is a LOT easier than the chemo your mom is going to go through. Keeping her sense of humor will definitely help.
I used to treat my mom to a spa pedicure when I would visit. Even though I lived over 100 miles away and didn't have a car, I would managed to make it once a month. She had never had a pedicure before this and fell in love with the pampering she got from the knees down. We also used to play "beauty parlor", where I would wash her hair for her and blow dry it in different styles just for the fun of it.
Dad was tougher, because all those froo froo lady things didn't appeal to him. But I used to go with him to the hardware store or the bank help him fix the sprinklers and all the little handyman/woman chores. Just he and I spending time together was comforting for us both.
One thing I just thought of, but never did myself...when your mom is feeling rather run down from the chemo, someone might read to her from a book that she loves. I know for me, when I am having an especially painful day and it hurts to even hold a book, my fiance and my kids before that, will read to me. It is so relaxing!
In essence, just spoil her rotten. Fix her favorite meals, rent her favorite movies, buy her some elegantly smelling bath washes or french soaps. If she likes to wear makeup, have someone doll her up so she feels pretty even though she may be feeling kind of yucky on the inside.
But most importantly, I think the best thing you can give her is a gift of your time. Just being there as you already are.