For water-heating I made a plan, I dare to link it here too:
For heating I plan to make a beer-can solar heater that I can put on top of their coop, ventilated by computer-fans.
A separator "wall" will be installed to reduce "splashdamage" caused by the drinking water to the much deeper layer of straw they have in warm season. Also the water will be put on a box with multiple holes, I really do not want to be the straw wet around the waterbasin. I bought lots of straw for this winter and I will use it amply in their yard too.
Aaaaand: dry sauna! Ok, not the real thing but basically the same idea: in the house we heat with wood, and the iron fireplace has a closed top where I will but basalt stones, heat them up for the night. I put those stones in a metal lined wood box (all sides closed, big enough for a duck to rest on top of it) which I put in the coop.
The hotbox emits heat, but it is not warm enough to light up straw.
A hot bath is not easy to manage (and it is drinking water heated up, not cheap), but I will manage to move hot water out to their pond with a pump and hose, just to see them playing in the steam. For afterbath preening only they will have an infra lamp, but I do not plan to run it 0-24.