Since there really isn't much at the airport in Kansas City, take the shuttle to the Marriott and sit back and enjoy a meal there after you check in to fly. This is a great way to kill an hour or two if you are early (like I was) and not have to get a car or eat fast food.
I stay fairly often at the MCI Marriott. It's been updated. Upgrade for platinum is almost automatic for platinum. Nice concierge lounge. Good room service. And as pointed out, it's great if you've got an early morning flight...takes about 5-10 min on the shuttle to your terminal (depending on whether it's A, B, or C. And, they still offer the full breakfast buffet to my wife and I on weekends when the lounge is closed. I think you'll like it.