In our house, we have a practice of letting shower water accumulate in the bathtub, and just letting that sit and dissipate heat until it gets cold. I have no idea how much energy this saves, but it feels like it ought to be some. Possibly some humidification benefits too.
It sounds like you want something that you can use to heat a bed or something, though? I'm not sure how I'd approach that. Maybe a stainless steel water bottle? I'd definitely want something that was very easy to clean.