Will Grandma be in or have access to a wheelchair? I think a great option is to get a bus at the Juneau dock and head to the Mendenhall glacier. That's a sight everybody visiting Juneau should see. But as I recall, it's uphill from the parking lot to the viewing areas and the visitor center is even higher. Wouldn't be too hard to push a wheelchair around there though. If she can walk some by just taking time you could pull it off that way. Another highly recommended Juneau adventure is whale watching. No walking involved in that. It's great to watch these magnificent animals up close. You can do both the whale watch and Mendenhall. You'll have plenty of time. In fact on our whalewatch trip they dropped people off at the glacier on the return trip.
In Ketchikan, a non walking event that we found quite fun was the Duck tour. A nice tour of the town and then you drive into the channel. They pick you up at the dock. Again, with a wheel chair Ketchikan is an easy town to walk around (and push).
We didn't do any tours on our visit to Sitka. It wasn't one of our favorite ports. I don't know how much walking would be involved but I hear good things about the raptor center. You can buy a buspass for going around Sitka which would take you there and to other spots.
Enjoy the trip....you all will. HAL will treat you very well.