NATO's recently reaffirmed policy of nuclear sharing means that the US loans out nuclear weapons to trusted members of the alliance. The US maintains the weapons in peacetime. During war conditions, control goes to the government where the weapons are stationed. One response to the Ukraine crisis is to extend the nuclear sharing program to NATO member states like Poland and Romania. The smaller and weaker the member, the more aggressive the response will be perceived in Moscow and the greater the risk of problems with the deployment.
Poland is probably the safest bet but it's also the least threatened. The Baltic states are the most in-your-face option and Romania is probably the most on-point and the most subtle of warnings due to the interplay between Romania and Moldova.