They do take a mild interest in growing things.
I do keep an eye on Freecycle - latest joy was a basketball hoop that doesn't stand up
What they would love, of course, is a large climbing frame or trampoline, but there's no way we can afford one
I was wondering about things like big tyres for jumping/balancing on. Maybe. Don't know if it's possible to get old ones free.
Or simple log constructions - balancing beams - possibly. But logs are expensive too I suppose.