I made my wife count calories and lift weights before she got fat.  Getting her to do real lifts was a challenge, but she loves it now.    If you need to convince your partner, keep reminding them it's for their health and happiness, then invite them to join you for physical activities outdoors and significantly outpace them.  When they get all tired, tell them "see, this is why I want you to work out with me, so you can have fun like I am!"