First it was a stick he wanted to be, so he could sail down the sewer into the harbour.
Yesterday, he tells me would love to be a toy car so he could get played with on ramps and get swung on ziplines. However, he has a stipulation. He would not like to be a car who gets played with by a baby. Likely, he was heavily influenced by Toy Story 3 where it is made clear that babies play with toys much differently than older kids. Babies don't understand that toys don't want to be thrown around willy nilly.
For one thing, babies don't know how to build ramps.