I would have to go out! I am always happy to help friends out with childcare and often have a couple of extra kids BUT I can't cope if we stay home all day!
I'd get them to bring scooters and go to a local green space, park or similar. If you honestly don't have anything in walking distance I'd do a penny hike type activity, or take every second left turn til you're home for eg. Mine love checking on Google maps and seeing where we are. You could take a checklist of things to see while you're out - red car, chimney pots, birds on a wire, pink flower, a double door number (11,33,77) etc etc
If you can get somewhere go to a library, museum, nature reserve etc for free or cheap entertainment.
Go on a bus safari, just buy a day rover type ticket and journey around, find a double decker if possible
Home based activities -
Make your own food, so bake bread hedgehogs, make soup, make jelly, bake a cake, make pizzas. You can lay out 'stations' and pre prep a lot of it. You could so those cupcake kits so the decorations are premade.
Let them dress up - using dressing up box, your clothes, their clothes, get them to dress up for a 'disco' then play music and games.
Play 'libraries' - get them to peruse shelves in your house and pick 2 books each, then sit and read them together and act bits out etc
Have a picnic on the floor - get them to ring soft toys, lay out blankets, choose the snacks and then sit and eat them.