While you have a walker and wheelchair if needed, I would really (really, really, really) recommend renting an ECV (motorized scooter). Why? Because
Disneyland isn't flat, and pushing a wheelchair is going to suck. When added to the current heat wave (81º today, 83º tomorrow, 87º Monday, 88º Tuesday, and 90º Wednesday), I would do anything I could to avoid pushing a wheelchair!

You can rent an ECV at the wheelchair rental in the Disneyland Resort Esplanade, or there are a few places that will deliver to the hotel so that she can have it to/from the parks as well. Deckerts, I believe, is one place located right near the parks on Harbor Blvd, and they are a bit cheaper. Seems like I have heard something about Apple Scooters here on the disboards, too, with good reviews.
Of course, along with the heat and general good-health maintenance, remember to hydrate well! I know when I'm at Disneyland, I'm having so much fun that I tend to forget that it's been hours since I last had a drink, because I just wasn't aware of it, since I wasn't in my normal element. No biggy for a relatively healthy 30-something year old, but a little more important for someone in their late 70s!
Now, for the fun stuff! I'm thinking of things my Mother-In-Law/Father-In-Law (early 70s) would enjoy most, and it would be things like....
Great Moments with Mr Lincoln (FIL is a history/presidential buff)
The Gallery right next to Great Moments with Mr Lincoln (lots of history stuff)
The Disneyland Railroad (the dioramas are one of my favorites! Plus nice and relaxing)
Storybook Canal Boats (MIL loves miniatures, the landscaping is amazing as well)
The Golden Horseshow show (not sure what it is now, but I've loved the past ones)
Walking through New Orleans Square, especially the back alleys
The Mark Twain
The Aladdin show
Beuna Vista Street and the accompanying shows (great 1920-30s theming!)
Of course, she may also enjoy the more typical rides as well, but these are the things I would make sure to hit that an older adult may find extra fun/neat/interesting.
Enjoy your time with your mom. I'll admit, I'm insanely jealous of you right now--I lost my mom 7.5 years ago (when she was only 56), and we never got to take my daughter to Disneyland together, which she would have loved so much! She was the main force that started my love of Disney.