I will be the first to admit it. My children have WAY WAAYYY too many toys. The kicker is that most of them only get played with a few times before they are tossed aside and ignored for weeks, months, even years at a time. There is one toy though that has persistently been used time and time again by Frugal Boy. I used it as well when I was a kid. It costs nothing and takes up zero space. The more it is used the better it becomes. There are no batteries that need to be changed. It can be used anywhere at anytime. Every child in the world already has it. What toy could possibly check all of those boxes? Imagination. Imagination is the only toy any child needs.
Kids can use their wonderful imagination to keep themselves entertained wherever they may be. It is always with them, never gets dirty (that comes later in life), and is wonderfully adaptable. Here is Frugal Boy jumping off a tall cliff into the sea.
A little bump in the topography is all that is needed to be King of the Mountain.
A small drainage stream is HOURS of entertainment for an active imagination to work with.
A pile of busted up concrete provides a wealth of possibilities. Frugal Boy was happy to imagine himself as some big shot builder.
Toys that you buy at the store can help an imagination, but when that imagination has extracted everything it can from a particular toy, it gets tossed aside. That is why toys such as Lego, blocks, and dolls have such a long play time. They are good enablers of imagination letting the child play out a wide variety of scenarios. The more your child practices using their imagination, the better they will get and the fewer aids they will need.