Actually, this is why it's such a great business opportunity, see you have to buy a 2nd robot that cleans 1st robot and then a 3rd robot to clean the 2nd and so on.

More seriously, Stretch actually looks pretty straight forward so I could imagine a combo-charging/dock/cleaning station where it can swap tools (ie, grab a vacuum, drop off mop, etc) and get cleaned by the cleaning station. Also once the robot is in a more finalized version, its not unthinkable to make one that is waterproof enough to go through a carwash type device.