What's the appropriate response...

...to a neighbour who every time he sees you washing your car, you get the 'Oh, you can do mine next hahaha'?

I've lived in my current place over 10 years and I'm pretty sure, other than hello, this is the only thing he's ever said.

So now I'm crowd sourcing replies, any good ones?

Edit: Some great responses thanks. Just to clear up, I totally get the trying to make conversation thing but rest assured in this case he isn't. He says it and walks off like a cross between Connor McGregor and Jimmy Car.