My mother suffers from Alzheimer's and some other conditions. She recently spent about a month in a skilled nursing facility, doing physical rehab and strengthening before returning to live with me. About the time she was admitted, I purchased an HP touchpad for her as a gift. I think that the device was great for her, but not in the ways I expected. She used it almost daily. I was able to load it up with a lot of music, which she enjoyed greatly, And the slideshow function also received rave reviews from her.
However, the touchscreen interface (though seeming simple to me), was not ideal. First of all, it proved difficult for her to master the touch interface because of problems applying enough pressure on the screen, or too much. Those of us without arthritis, and who do not have as much difficulty with manual dexterity, perhaps do not always anticipate what a challenge it can be to master the making of gestures to interact with a touch interface. Second, learning the GUI itself proved hard, and her preferred approach became to call me on the telephone to have me make sure that the programs she preferred were running. She was pleased to have music play on on a preset or shuffled playlist, but with a month to try, it did not seem that she managed to master how to select particular songs off of the playlist. Switching between programs was a task on which she would make progress with coaching, but repeating this skill on her own was a real challenge.
I've also let her work with my iPad, and similar difficulties are present.
Your grandmother may do much better, and may not necessarily find it quite such a challenge. But there's also a good possibility that she will find efforts to operate it without the assistance of others, rather frustrating. I recommend spending time together in person, and augmenting that quality time by using the device as a conversation piece, something to talk about and an activity to share together. Perhaps there are other devices that are more suitable, but I do not recommend touchpads generally, except for use as a common activity to enjoy together.
Good old-fashioned books, with pages to turn, on the other hand, are a killer app for all ages. My mom cannot get enough of them. And I personally believe the stimulation is very important for maintaining cognitive function as long as possible.
Hope that helps.