Waterbed, dude. Waterbed. I've had one for 20 years. They stay cool in the summer, and you can turn up the heat in the winter.
I don't even think that they make those sloshy 1970s "big bag of water" kind anymore. They're all waveless now. I mean, I think they are. I still have the same one. It's the one mattress that never wears out (although the heating element will, after about a decade. Those are easily replaceable).
!!! interesting
I admit, it's hard to think of a waterbed w/o the slosh...