Ectothermic is almost always used in scientific contexts.

#Snakes are ectothermic, or cold blooded, relying on environmental heat sources like warm rocks, soil or roads to stay warm. #NatureFacts

— Archived: USFWS Midwest Region (@USFWSMidwest) November 17, 2015

Since they don't sweat, a gym for snakes wouldn't smell bad, however it would have to be quite warm so that the snakes (ectothermic) could be active.

In conclusion, those are my two and literally only thoughts on the subject of a gym for snakes

— Ryan North (@ryanqnorth) August 20, 2018

To be honest, the weather has been so good for so long I'm considering becoming ectothermic.

— Jules Howard (@juleslhoward) July 13, 2018

Did you mean exothermic?