I think that the older you get, the more you enjoy new experiences rather than a 'thing' - if you know what I mean. How about some pampering at a spa, or perhaps theatre tickets. It all depends on your mum-in-law's health/mobility of course.
If she is anything like mine and loves to have family photos all over the place, a digital photo frame will be a fab gift. You can put LOADS of photos on them and it displays them in various modes that you program. Your m-in-l will just have to plug it in and enjoy it. You can even put short videos on them. Looks fab and a she will be able to show off all those photos to her friends. Try to get hold of her wedding photos and old albums and put them on it for her.
For her 70th we got my Mother-in-law a bird feeder, one of those poles with 4 arms hanging off it off which you can put feeders containing seeds, nuts, fat balls etc, plus all the feeders, some seeds and nuts etc. She's got it sited near her conservatory so she can sit and watch the birds.