Go for it op - there are some really miserable people on here trying to impose their own misery on to you for this - if you think that your gran would like it the do it and enjoy it.
We did this for my mum’s 80th birthday and she was really touched by it. We also didn’t get her any tat or spend loads of money on it.
We decided on what our main presents were going to be (from me and said) - she likes expensive perfume so got her a gift set that had travel sizes of shower gel, cream and talc in a bag for free with it - so that was 5 presents, all used and appreciated and what she likes and would have been considered a normal nice single present. Also she had been dropping hints about some nice earrings and necklace she had seen so that made three more presents that would be considered one normal big present.
She plays golf so new balls, golf gloves, tees, golf pen and clip on towel made up another 20 or so things, easily put away in her golf bag, all used and appreciated because they need replacing.
She loves some chocolates that come in little bars (think like kinder bars or the Green and Black ones - two boxes containing 10 bars each - another 20 items that went by her bed and savoured one a night, would have lasted less than a month and been considered a treat as she doesn’t often treat herself to them.
A bag of 20 marbles. We have a long running joke that she’s worried about losing her marbles so this was to replace any when she starts losing them - maybe considered tat by some on the thread but provided lots of laughs and as she used to enjoy playing them when she was young, she taught her grandchildren to play against her and they all had a lovely time and made lots of happy memories together, they live in her games cupboard and still come out at family gatherings to be played by all generations so well worth it on several levels for the fiver they cost (making it more like 25p each for these 20 presents).
Another thing I got her was a penny from her birth year - they cost about a pound on eBay. Thinking of ‘see a penny, pick it up, all day long you’ll have good luck’ - she was very touched, put it in her purse and was upset to lose it - to the point she asked for another one to replace it.
The last few bits were made up of some special bulbs for her garden, a rose that had a meaningful name, a glass nail file so she could have one for her bedroom as well as one in her handbag and some new nice biros - think a pound a time rather than a bag of 30 from Sainsbury’s as she had used one her sister had and really liked how nice they were to write with. All very much appreciated and used/planted and enjoyed.
Not saying to go for these things exactly but to show that you don’t have to spend a lot of money to do this, you can still get a nice ‘special’ present from you and/or your mum too, and have not involve tat or clutter but have a lot of fun and make memories with it too.
Mum was adamant she didn’t want a party which is what we had thought about doing initially (some people would love it I know but not all) so we all went away together, I took lots of photos and one of the presents was an iou for a photobook of the weekend which subsequently gave her lots of pleasure. So you can still do a book of memories or wishes from others - the 80 presents is completely different and can be enjoyed as much as the other.
Hope that you get 80 things you and your gran love and she has a great birthday!