A YouTube video showed slime-play with a tennis racket. I didn't have one, but a potato masher worked, to create strings of slime.
"Bath toys" are sold in stores, but kitchens are full of things that are fun in bathtubs. Colanders, measuring cups, mixing bowls, slotted spoons to pick up bubbles (or blow them). Ice is great in the tub, and cheap, and cleans itself up.
I think one thing that helps is having a house. A detached house.
. . . .
It's not a requirement, but it seems to help. Then kids have dirt to dig in. I know some apartments have dirt and some houses don't. But still. Dirt. Bugs. Plants.
There is more of that, and more about what else helps, in a chat transcript: SandraDodd.com/chats/whathelps photo by Lydia Koltai