Pinterest has endless inspiration.
Have a look through all your children's art ideas books or forgotten craft sets - there must be something they're not attempted
Have an indoor disco - draw the curtains, turn on flashing fairy lights, crank up the cheesy pop
If you don't mind your mattresses utterly destroyed, and your stairs are wide enough, line them up top to toe going down the stairs with a big double one at the bottom. Let them play 'mountaineering' all day.
Teach them how to cook. If they're old enough they can play at 'restaurant' where you're the customers, taking it in turns to be chef and waiter/ess - one makes the main meal (can be cold food, or something very simple like scrambled eggs on toast, that will still give them a buzz) while the other 'waits' on you (sets the table, takes your order, pours your drink, brings the food etc). Then they swap roles for dessert. It'll make them feel so important, even if their fabulous dish is a scoop of icecream with some chopped fruit on top. You can actually spin this activity out all morning if you have them writing menus, and making table decorations.
A sensory tray with coloured rice is a big hit for all ages, and will keep indefinitely if you store it in a plastic tray with a lid (fits well under a bed). You'll want to spread out a sheet or 'smooth' blanket (that rice can't stick to) underneath, as it does go everywhere.
Making puppets and putting on a puppet show. Start with simple nursery tales and then see how imagination unfolds.
Treasure hunts around the house and garden. Children love searching for stuff! Stick wrapping paper down on old cardboard and have them trace shapes and cut them out. These will do to hide. Or give them a shoebox and tell them to go and fill it with as many different things they can find that are orange, or begin with a 'B', or are made from wood, or whatever. Set ground rules first so they know no raiding your room, no live insects, or whatever
On a warm day, make a 'waterwall' in the garden. Raid the recycling for plastic containers you can use to cobble something together, and hunt in the garage - there's nearly always some random guttering or pipes in there!
Play hospitals. Make several beds for patients (teddies) and dig out any doctor's stuff you've got. Raid your first aid kit for expired bandages, and the stash of calpol syringes that every family has. If you're the paramedic you only need involve yourself every 20 minutes or so by rushing into the room making a siren noise and handing over a new teddy with a quick explanation of what's wrong with it, then you can get back to what you'd rather be doing.
Make a zipline for barbies out of a plastic washing line and a bulldog clip
Make giant bubbles (look online for info)
Have the children act out a favourite picture book. Again the whole day can be spent making costumes/learning lines/finding props etc. They'll fight, of course, but children do that!
Waterplay in the bath, as long as they like. Offer some random silicone kitchen utensils or jugs or whatever that they'd not normally use to add interest.
Sort through old toys and clothes with them, Marie Kondo style.
Have them make their own jigsaw puzzles by sticking a large picture from a magazine onto cardboard, then cutting them into (not too small!) pieces. Then they can swap and do each others puzzles.
Make slime, if you can. I was never able to master it!
Make a den and get in there with them to read.
Make an obstacle course.
Play 'egg and spoon' races with potatoes. Practice other sports day races and games with them. If you have a wide age gap so the eldest has an unfair advantage, tie their shoelaces together so they have to hobble very fast instead of running, or set their finish line twice as far away which gives the little one a chance!
Talking of which, have them do a 'beep' test! Did you ever do one at school? If not, google it to see what I'm talking about. You can get the 'beep' noises on Youtube. If they're little, make the distance shorter.
Have them wash the car, and hoover the inside too.
Make bird puddings and have an unofficial 'big garden birdwatch' (you can make binoculars out of toilet roll tubes). Dig out any bird identification books you have. Let them take photos and keep a tally score of how many of each kind they can spot. Also stress the importance of them being nice and still and quiet while they watch (so you can sneak off and have a cup of tea!)
Role play something they never have before. Shoe shops, or jungle explorers, or secret agents, or cave people, or zoo keepers.
Crank up the music and get everyone dance-cleaning.
If you live within striking distance of the beach, make full use of it!!
Also bikes. Use your bikes. A lot. Every day.
When we were little we'd go around with a tape recorder recording random things, then have each other try to guess what it was - the loo flushing, the fridge door opening and shutting, the hamster in his wheel, etc. I'm sure something similar can be done with a smartphone!
Have a 'parallel lives' day. Pretend you're all your secret unknown cousins. Everyone gets a new name and identity, interests, all of that. Maybe your all have to talk extremely posh? Dress up in the most unused clothes you can find and let the kids create bizarre hairstyles for each other (and you if you're brave!). Eat food you'd not normally eat.
Have an indoor picnic. Always fun! Use biscuit cutters to make shaped sandwiches.
Have kids make fruit kebabs, and let them drizzle them with melted chocolate.
Dig out any unloved board games and rediscover them. If it turns out there's a reason they're unloved, you know to rehome them, otherwise they might be better than everyone thought!
Have them make a magazine! That'll last days, that activity.
Get them to help with the gardening. Grow stuff.
If they have a favourite TV program, have them work together to write a new episode for it.
I don't know how it's done or how easy it is, but I know a little munchkin who enjoys making 'stop motion' videos of her playmobil characters.
There, that's it, I've run out of ideas!