There's not much he can do without giving china and russia an excuse to act crazy. Technically, America can take care of itself so there isn't anything it needs from china, russia or korea. Aside from korea's proximity to the U.S., the real problem is china, which is in league with russia. That being said, what can President Trump really do with korea that wouldn't impact relations with those two major countries?
Let's just hope that President Trump has no connections to the progenitor of the georgia guidestones!:
"1- Maintain humanity under 500,000,000 in perpetual balance with nature."