@mattly do you already have a calendering or checkboxing practice? If so, I'd consider going that route. My general approach to building habits is trying to chain them off of an existing one. If there's something else in the vicinity of the gym that you/the family likes that could be an after gym treat (the library, or, I dunno, smoothies), that might make the cause/effect of the reward for going to the gym more effective.
@KatS@jasongorman this reminds me of a Kernighan quote on writing clever code:
"Debugging is twice as hard as writing the code in the first place. Therefore, if you write the code as cleverly as possible, you are, by definition, not smart enough to debug it."