Though there are many ways to do so, authenticity is key, he told benefits and HR professionals.

"If you do this you will create an awesome office," he said. "You are going to create massive productivity. The war for talent is over; they've already won. Do something so that your best employees don't leave."

Here are four tips from Kelly on creating an awesome office.

1. Focus on inclusive health.

This means giving just as much attention to the mental health of employees–and maybe more–as you do to their physical health.

"You need to ask yourself, 'What is the mental health of our team?,'" Kelly said. "Mental health is a prerequisite to our physical health. It's amazing how much we focus on fitness classes rather than focusing on happiness and fulfillment [of our employees]. It's got to be top down; it's got to come from the executives."

Employers can find a way to make their employees feel fulfilled–even such small things as offering regular encouragement, or doing things to make them happy, such as having creative days at the office where workers paint or create.

Read: Employers rebranding, refocusing wellness programs

2. View onsite food as an engagement tool.

Yes, the way to an employee's heart is really through his or her stomach. In fact, a recent poll by Peapod found that those who are offered free food and snacks on the job report higher levels of job satisfaction than those who aren't.

"This stuff matters," Kelly said. "If you're not offering snacks, if you aren't offering things to eat at work, you will fall behind. Millennials especially care about perks like this; if you don't offer it, they won't work for you. It's not just about perks, it's about knowing your company cares about you."

Some ways to do it:

• work cafes, which can create a sense of community

• free (and healthy!) snacks in the breakroom

• healthy market or vending machines where employees chip in on the cost

"You want your employees to be productive and happy and be better members of society," Kelly said. "So why not give them the fuel to do so?"

3. "Exercise to feel great, eat to lose weight."

Employers, and employees, need to change the mindset from thinking of exercise as a way to lose weight and start thinking of it as a way to feel great instead.

"Exercise is about how you view yourself," Kelly said. "It's so much easier to go take a 10-minute walk to make you feel better rather than having to do something a thousand times to make yourself look better."

Employers can help change the mentality by even making small changes:

• incorporating fitness stations in the office (rather than an onsite gym)

• creating a track using tape on the floor so employees can move while visiting or have walking meetings

• organizing company hikes

• offering yoga classes in the office or healthy cooking classes

Read: 9 tips for creating an obesity program

4. Leverage social chemicals to your advantage.

"The best thing about social chemicals is that it's effective and it's free," Kelly said, citing serotonin (when you feel important) and oxytocin (when you feel loved and trusted) as the two employees need to experience at work.

Bottom line: The key to having a happy, productive workplace is about making your employees feel important, valued and trusted.

"It's tough to recognize everyone; time is crunched," he said. "But you need to make that time to do so."

Every Friday, Kelly said his company has a meeting where the employees say what they are most grateful for and then call out another employee who has done a great job that week.

"The important thing is to make sure your company feels like a community," he said.

Read: Employers beefing up employee recognition programs