I like what LaRue’s already said. I also love the idea of families exercising together. I don’t mean the kind of exercise that leaves them exhausted and crabby (unless BOTH partners enjoy that kind of thing), but making habits of the things they already enjoy. Take a stroll after dinner & chat together, go for a bike ride or a hike. If the more reluctant partner of the two has any interest in a specific sport or activity, I would encourage their significant other to jump on the bandwagon! Even if they only watch baseball or tennis, find activities that could relate to that interest (go to a slow-pitch batting cage or walk to the nearest card shop). Anything is better than sitting on the couch.
One final thought to go with not shoving ideas down their throat- don’t sneak them into anything. Showing up at a park where they “happen to have a bootcamp” will go over about as well as swapping the hamburger for turkeyburgers. If they’re not willing to do it themselves, they’ll only resent you for forcing the issue.