Every morning when I'm taking ds2 to school, we pass a nursery and we can see right into their outdoor play area. They always have several of those plastic bakery crates scattered around outside. The children play on them
There's also some used tyres.
I don't consider it safe for children of that age to be playing with stuff like that. The bakery crates are made of very hard plastic which could injure if they were to break. They're not intended to be used as play equipment.