You could do a scavenger hunt. Hide things around the house and call out the items for the kids to find. My mother used to hide little honey butter sandwiches when we were small.
My mom often had us sit down and listen to her read before our naps. That tended to get us 'unrevved' so we could relax and nap. I don't know if most parents these days still do an afternoon nap but it sure made us less crabby and tired later on in the day.
You could 'do' an adventure. Encouraging kids to act out a movie or cartoon or book. Some old clothes, a few props, easy stuff.
My mom once got a couple live lobsters on a rainy day, LOL, we were playing with those things til we were exhausted, LOL.
You can make play dough out of flour and a couple other cheap ingredients, it's easy to clean up, too.
We took a pottery class at the museum when we were very little, that was also free or very cheap. Museums in general do a lot of low cost to free activities for kids. But keep an eye out on the internet. There are tons of free activities for kids around and about. Book stores often do 'story telling time' and there are tons and tons of things out there to do.
I do still occasionally do 'kid days' though my health doesn't allow it much. My friends dropped off their kids at my house and I spent the entire afternoon exhausting them, LOL. Playing with the animals, cooking, things like that.
Kids LOVE to cook. Creaming butter and sugar, kneading bread, cutting out cookies, all good stuff. Frankly I'm amazed at how much kids can do. I used to have a couple 'kid proof' cutters for slicing vegetables, too.
One thing a friend of mine did with her kids was 'Mess Day'. They LOVED mess day. She did this in her barn but a shed or garage is fine. She took a couple buckets and filled them with pebbles, all sorts of objects, clay, water, mud, and the kids would stomp, squish, squeeze, throw and slop the hours away. She did a similar thing with washable paint. Put a big piece of paper down and the little monsters walked through the paint, put it on their hands and did all sorts of things. Cookie cutters (cut a design on a potato then use it for a stamp), string, and sticks to splatter paint with. If I recall she suggested a 'Run Naked Through the Neighborhood' day too. She would do ANYTHING to tire them out, LOL.
Another possibility is to let them use an old shed or garage temporarily to put up a tent inside, and make a 'fort'.