Snowden was looking for notoriety, and perhaps avid for U.S. defense secrets to divulge. In the process, he caused harm to the United States by revealing classified information despite his oath of loyalty to his motherland. The law should take care of punishing him for his actions. That would involve some diplomatic complications, with none other than Snowden´s host Vladimir Putin, whose inflated ego will dictate the outcome.
Ignoring the issue would send a strong and significative signal to the American people, to the international community, and to Snowden´s pals at the NSA that the treason route is open for business.
The option of pressuring Putin for the release of Snowden to the United States is there for Obama to undertake, though no one would expect Putin to ever agree to that course. Other countries would probably stay away. I think the United States should use any leverage at his disposal to persuade the Russian president to agree to Snowden´s release. However, Putin´s ego is way too inflated to agree to that.