I mean, you're not locking them in a sensory deprivation chamber, right? Your kids have sometime in their lives expressed hobbies and interests, and they may have some things that cater to those hobbies and interests?
Available to my kids: books, library cards, Lego, art supplies, more books, games, puzzles, paper and pens to write a story, toys to make up stories around, playing cards, marble run, electronics kit, science kit, other stuff. We're lucky enough to have a tiny garden so add basketball + hoop, football, a bit of grass to play on/lie on.
Stuff I've come across my bored kids (4 years apart) doing: Parliament of stuffed toys, science experiments making 'potions', youngest makes almost homeopathic fruit juices by mixing trace elements of a raspberry in water, they both like making potions/seeing what dissolves/playing with baking powder, they'll bake a cake or biscuits with a small amount of supervision (but not me coordinating the activity/jollying them along - I'm there for advice), recreating movie scenes in Lego, making their own superhero movie, reading, writing stories, playing/making up music, occasionally they'll even do something 'wholesome' and straightforward like play a board game or card game together. I sometimes have to intervene when the basketball gets heated or whatever. And often I'll join in when asked but I'm not there filling every waking minute for them.
They're not particularly exceptional kids and it wasn't particularly easy when we decided to have stricter limits on screens. Youngest still whines 'I'm boooooored' two minutes after the iPad goes off. But ten minutes later he's just getting on with stuff.
That said, my kids also do a fair amount of clubs and stuff by choice.