Do the model engineering fanatics ever build heat pumps? All I can recall
seeing are various engines.

I was trying to think up a project that would not look too eccentric and
would entertain people. It occurs to me that you could build a little
hand-cranked heat pump that would chill a beverage can from room temp down
to near freezing. Since that represents only about 50 BTUs it shouldn't
take that much muscle in a properly engineered device. It would all be
belt-driven from a hand crank, running something like a Procon pump to
compress a refrigerant, with a belt-driven fan over a condenser, and an
evaporator that was a saddle to hold a can sideways that was slowly spun.
The saddle could contain a water bath to make an ice bank to chill the can
without freezing it.

Or maybe just a simpler version to blow some cold air in your face when you
crank it.

Or what would be even more impressive would be a gadget that made cold beer
on one end and hot tea on the other.