Pass a law which makes it mandatory for all citizens between the ages of 18 and 25 to serve 18 months of volunteer service of some kind. This could be with a government sponsored program, a not-for-profit organization, or a branch of the military.
Why is this important?
In years past, World Wars and the military draft did a lot to familiarize and equalize people from different ethnic, social, and economic backgrounds in this country. I believe our sense of an overall American ethos would be regained if we instituted compulsory service of some kind for everyone.