Offseason leadership training and other intangibles. Leadership, work ethic, teamwork, etc.
Laugh all you want, there is a reason fortune 500 companies invest heavily in these.
As a guy who has attended many of these and led more than a few, they are typically designed to teach people how to get along in a large organization. They are great for the bottom 60% of the organization, but often have the effect of neutering top performers. It just doesn't seem like a good fit when talking about football players.
One exception, and perhaps this is what you intended, would be team building excursions when the intent is just to let them build bonds by having fun together. Whether paintballing, white water rafting or some equivalent activity, these can be used effectively to bring an organization closer together.