I read on an aquarium website that the most humane way to dispatch one of their little buddies was to immerse in an ice bath. That is water ice mixture. Adding salt serves to make the mix all that much colder. This gives you a dead fish pretty quickly. It does not however guarantee a no flop cleaning operation. Crappie however are an odd fish in that they will flop about long after they should be permanently still. I imagine this is simply nerves twitching, but I like them to lie real still for me, too.
If you want to wait to clean them later, drain away the water leaving just the ice.
I have always just hosed the fish off to remove slime. Wanted them as clean as possible. Of course I kill my chickens before I pluck them, so there is that.