If you're not sure where they like to shop, there are VISA and Mastercard gift cards that are accepted everywhere so they can use them to shop, go out to dinner, use them online. I've given up trying to shop for gifts that will please, and now that the g'kids are grown, I do my "shopping" at the credit union and give them cash. Seems impersonal to me, but it's what they like to get. My DD/DSIL and DS/his partner get cash, too. They're at a stage where, like me, they mostly have what they need or want and "stuff" is just, well, more stuff! When they ask what I want, the answer is always the same: chocolate and gift cards to the garden center.
It was hard to buy for my MIL because she, too, already had everything she wanted or needed. As she got older getting out to shop was increasingly hard for her so we got inspired one year and bought her an assortment of greeting cards for birthdays, weddings, anniversaries, even a few sympathy cards, and a roll of stamps. She told everybody she knew what a great gift we gave her because it saved her trying to get out to get cards and stamps and get them mailed on time.
Another thing she liked was jam, jelly, cheese and sausage from the Swiss Colony because they can be bought in really small sizes that are just right for one person so get used before they go bad.