Boredom

My DM is the same, I sometimes think she enjoys the misery, and complaining gives her something to talk about

I bought some activity books for older people which they like doing. Aswell as some colouring too.
There's only so much you can suggest if they don't want to do it.

Could you invite some of their friends around for tea at their house?

Can you give them any chores to do? 😆😆...... Keep them busy, helping you out a bit..... Go to the post office for you etc.

Family tree is good.

How about setting a reading target? When my mother retired, she announced she was going to read all of Dickens' novels, in chronological order. It became her little thing. And we all played along, asking her where she'd got up to etc. A reading challenge (I mean something quite demanding) could be very rewarding. It doesn't have to be Dickens, of course (they could plan to read all of Shakespeare's plays, or the complete works of Virginia Woolf, or Thomas Hardy or Jane Austen).

Buy them a telescope and get them into astronomy.

Encourage them to learn a language (something unusual, like Russian or Japanese).

Could they learn a musical instrument?

Yes and it's bloody exhausting. I'm wondering what they say when you ask "Well if none of those are suitable what do YOU want to do?" Because I found that doesn't work either. Some people seem to get in a rut of misery and find excuses to stay there.
That being said I do have huge sympathy with elderly people (if they are) being petrified of covid.
Have you suggested walking groups or guided walks around the grounds of places? Something maybe in the fresh air where they'd feel safer.
My MIL watched rolling news 24 hrs a day, it doesn't help that's for sure.

They need to create a routine for themselves to fill their days.
My MILs friend was told to do this after a period of depression. So one day it was the library, one day meeting a friend for a coffee, one day food shopping etc and then weekends off. She had to get out and go somewhere 5 days a week. Worked well for her.
Of course MIL was the same, no interests or hobbies, didn’t like to go out. Bored out of her mind.

I think it’s hard to start hobbies when you are older, especially if you have previously had no interest. Instead they need to create a structure to keep themselves busy.

They are grown adults, I'm sure they can think of things to do!

Library, go to town, get on a train with a mask to the outdoors, go for walks, book a holiday, get an older dog to give them purpose, lots they can do!

Yes yes yes😔
My mother, nothing made her happy, nothing was good enough - it was a nightmare for me, I tried my best but she was horrible to me, everyday I had to brace myself to see her.
She died last February at 92, it was a relief for me and for her.
Sorry no advice, I tried everything.
I think it will be something you just have to live with.

Put the problem back in their hands - I'm sorry to hear that, what are you going to do?

Oh family tree is a good shout too

Thats sounds very frustrating indeed. How old are they and how tech savvy? There are lots of interesting and free courses on Futurelearn. Ultimately I think all you can say is “I can’t think of anything else to suggest can you think of something you would like to try?”
As you say they are fit could they buy some home exercise equipment and compete against others online

Ex - Mil is like this. The family tree and finding new relatives kept her busy for a few years.

Local residents WhatsApp groups provide food for thought..... Cycle lanes, school traffic, cheeky people..... Keeps them going.