Dancing. I believe every lady in the world except for my mother absolutely loves to dance. Tell her that dancing will get her abs in better shape and that doing squats will allow her to "get low" and be able to have better, quicker dance moves and allow her to dance longer.
I think this should be the end all be all of "I want so and so girl/girlfriend/spouse to start working out" threads. Learn how to dance, or find someone to dance with her and break it down at your house or the clubs 2-3 nights a week for at least 30 minutes. If you go to the clubs moderate alcohol consumption so you don't mitigate gains (I know it takes some cajones to dance sober but it's not impossible...).