I'm an adult in my 40s with 3 siblings. Parents don't spend anything on us but they spend approx £20 on each grand child. It's too expensive and no one actually needs more stuff. If we did need it, I'd appreciate being asked so I could say I desperately needed socks, or a new steam saucepan.
Before each of us had children, it was £40 per family instead.
We are all grown ups now with jobs, and can buy or budget for our own things if we want to.
I think my parents and each of us were overwhelmed buying that much stuff and often we couldn't think of anything, or gifted/received items that perhaps weren't quite right or we were just exchanging money or practical items for the sake of it.
I think gifts that are generous with time are nice. Like arranging to babysit or have the grandkids to sleep over at your house (never happened for me but I'd love this present) or if you wanted to spend money booking the grown ups an experience (like a meal, an art class, kayaking, zorbing, canal trip, brewery tour, karaoke, bowling, flower arranging, trip to a show, gallery, museum, shopping trip, steam train ride, trip to an arboretum or lovely garden / trust etc).
I also think any meaningful gift was very gratefully received Like a copy of your favourite book for the grown ups, or a book you read to them as a child for the grandkids. Or a special item like a at Christopher's for someone who likes travelling. Or a simple mug with a nice meaningful quote about how much they mean to you. A soft fleece blanket, could potentially be personalised. One year I got a pound puppy cuddly toy and I love to cuddle it! I like anything that is sentimental or intended as a keepsake from my parents especially as morbidly I know I'll appreciate it when they're gone. Something personal like a handwritten copy of your family tree. Or your favourite recipes (or recipes theirs remember from their childhood). Or a photo framed or photo album of photos from yours and their childhoods. Memory jars (where you write down a number of memories on slips of paper - small stories from their childhood, how much you love them, your favourite quotes or jokes or advice, or could be anything really so they can read them whenever wanted).