My own Nan is easy to buy for. Sounds awfully stingy but I always buy her what I always have - nice hand cream, a diary and some Thornton's special toffee! Any deviation from this over the years has always led to a feeling of slight disappointment, so I stick to the same!
DH's grandparents have 7 DC and 20 adult GC. It is impossible to think of what to buy them. I've seen them bought so many, many things and they're so lovely they're always grateful, but put the things away and they're never seen again. They have on display everyone's wedding photo and the first baby/ school photo of each Great GC. Although they're given newer photos, these aren't what they want to display.
I'm sure everything these lovely people could ever want has been bought for them at some point, with such a large family buying for them.
Any suggestions that I may not have thought of? We rarely see them and they're too frail for outings, meals out etc now.