There was once a very wise user by the name of /u/AC_7, he wrote an answer to this question a while ago. Unfortunately his account was lost in the Great Askhistorians fire of 1918.

Here is a summary of the post:

Nothing concrete, there were attempts to build bombers that could attack the mainland USA from Europe. There was also the various U-Boat attacks and the various attempts to land spies/saboteurs in the USA. Hitler assumed that the United States would be unable to field armies right away and that they wouldn't be able to enter the fray until 1942/1943, which in Hitler's mind was more than enough time to deal with both the USSR and Britain. Hitler foresaw some grand struggle between the New World, led by the USA, and the Old World led by Germany. It was imagined that Germany would force England into a peace treaty and than the two would ally against the common threat in America; with Britain's navy being key to attacking the Americans

Hitler's ultimate goal was to establish German suzerainty over Europe and than lead this new German dominated Europe into the future. This was Hitler's goal, the United States wasn't the highest of priorities for Hitler.

U Boats were active off of the Island of Newfoundland (where I am from). I believe Newfoundland has the dubious distinction of being the only North American landmass attacked during WW2; a U Boat torpedo missed its target and hit Bell Island (off of Newfoundland's coast). What's really interesting is that the Germans actually landed troops and set up a weather station in Northern Labrador (the land portion of the Province of Newfoundland and Labrador). It was discovered in the 90s. Crazy when you think there were Germans walking around up there.