@SinisterBumFacedCat
Old diaries-going right back to when she was a child to just before she got ill.
Keep. My DH's Nan did daily journals her entire life. They're a treasure of family history and a fascinating look, not just into her past, but life in a bygone era. If your son isn't interested, his children may be.
Old school books and projects of hers.
Bin.
Projects she did for fun as a kid (little books/unfinished short historical stories)
Bin
Letters between my teenage parents (cringey but also very sweet) old long dead family members and friends she lost touch with over the years, plus lots of letters from an ex boyfriend.
Keep the letters between your parents. Bin any correspondence from relatives & people your son doesn't know personally. He's not going to care about letters from Great Auntie Beryl or Great Uncle Joe, let alone friends of his Nan's.
Letters and telegrams from my Nans correspondence with long dead relatives from many years ago
Bin, ditto re relatives/friends he doesn't personally know.
My Nans collection of greetings cards! Actually some of these are properly beautiful 1950’s Christmas cards I definitely want to keep
Bin, other than a handful of the most beautiful ones. Focus on those that were sent by relatives whom your son might know/recognize.
Film negatives of photos, no idea if I have the prints but it would be a big job to match up
Unless you want to tackle the task of figuring out what's what, bin. However, where I am (US) there are companies that specialize in digitizing photos from negatives to a CD or thumb drive. Not sure exactly how much it costs, but may be worth looking in to if you want to preserve them.
A crate of notes my Mum made whilst researching her family history. She produced a final document for everyone in the family with all the important information in. But this crate is full of folders of notes (all in pencil), maps, old documents that feel exhausted looking at.
Honestly, I'd probably keep these. Again, you could scan them (or pay to have them scanned) into digital format if you had the time/skill to do so. I don't think they're as meaningful as diaries or your parent's love letters which I think should be kept as originals.
I’ve also got 3 stamp collections I have inherited, hers, stepdad and my grandfather’s. I’ve no interest in stamps bar the arty looking ones.
Please get these valued before you decide to bin or not. My late dad had a random jar of old coins that we were going to divvy up to use as change. My DS1 told us to take them to a coin shop first. Turned out some of them were quite valuable. I'm sure it could be the same with stamps.
And yes, as a PP suggested, check with your extended family to see if they want anything before you bin. With a caveat that if they do want it, they have 2 weeks to pick it up or pay to have it mailed. We had a load of pictures of my cousins that I didn't want to keep. I ended up taking them to my old hometown when I next visited and my cousins had a field day.
It's really hard to get rid of a parent's things. Thank God I had a no nonsense brother who doesn't have a sentimental bone in his body. Otherwise I would have kept every scrap of paper and stick of furniture.