When we first meet Bing Bong we see a flashback of him and Riley sitting in his wagon which means that it’s a real item since Riley’s sitting in it. So how does it end up in her mind?
I imagine it’s like, the wagon is real but she imagined it doing things it couldn’t, and that version is in Imagination Land.
Besides, Imagination Land seems to gave some real stuff. Presumably she had real stuffed animals for her Stuffed Animal Hall of Fame, at the very least.
But the wagon was made into an imaginary rocket…except that Riley had to use cardboard for the wings.