A hot shower!
Re Hot water from engine waste heat. I haven''t tried it but would think two tanks in series might be an answer. Enter the potable water into one tank and thence to a smaller tank from which the hot water is drawn. Direct the hot engine water into the smaller tank first thence to the other tank in a manner counter to the flow of the potable water. The small tank should heat quickly and after a long motor the other tank should be hot as well or at least warm enough to avoid that sudden cold during the middle of your shower. Just a thought, George.