I have no experience in this but I'll give it a shot.
The keys to staying healthy are diet and exercise absolutely since they probably with through lunch it would be good to provide your employees a good and healthy, yet affordable lunch. It would be best to ask around to see what your employees would like and maybe set up a catering service for them or order to go from a reputable healthy restaurant, or just have your employees cook if you have the facilities available. Basically just have them eat healthier, gift cards to places like sweet tomatoes might help too.
Next is exercise. I can't help but think of that one 'The Office' episode where they are separated into groups and whichever team loses the most weight win something. If you already have groups at your company, either keep it, or switch it up, but don't put the overweight people together, and groups of 3-4 would be optimal.I like to think that people who don't exercise alone, would function better if they have a goal and have a group to encourage/punish them. But some might oppose as some just refuse to work out. Then there are those who already work out or those who just refuse to, and they can just opt out.
Just my 2 cents as I sit here in my lawn chair.