To clean the cassette while on the wheel--Get some sisal string--the old style natural rough coarse stuff- and use it between the rings. I have a chain cleaning tool and use that for the chain and if I am cheapskate it is sneak to the kitchen and use clothes washing liquid-- the less foamy stuff for automatic washers- and dilute with water 50/50. Then wash out with the hose and then to get rid of the water it is WD or similar.

Then it is down to clean the bike with soapy water and elbow grease.