It’s a hard phase this was my weekend life when mine were that age, I did also have PJs on TV days and pop a film on.
Buy them some play doh, (or make it) crafts, building wooden train tracks, rainy day walk, scooters, bike rides, bake cakes, local parks.
We had dogs as well so lots of dog walks in the woods and collecting stuff for the craft box and just busy days.
Move sofas or beds round and build dens with blankets sit and read books and get some coloring in books.
Honestly I could keep writing lists of cheap stuff to do with that age, I always avoided the play zones and inside stuff as well it’s all a bit manic.
What about swimming? Do you have anywhere that’s adorable near you? I used to find that was almost a whole day, with the obligatory chips after the swim.
It also doesn’t help that it’s a grey day, well it is here in Hampshire, always easier when the suns out!