Usually some form of identification, and definitely your passport. Most of the time, the only questions you will be asked are : How long are you staying? and, Are you here for recreational or business purposes?
yes
when does a us citizen need a passport to drive to Canada
When you are not a citizen of the USA a passport is usually required to enter the US. However, for many years citizens of the USA, Canada and several other western hemisphere countries did not need a passport to enter the US. Because of increased security procedures, even US citizens must have a passport or passport card to reenter the US across the US-Canada land border or by sea from the Caribbean after 18 May 2009. Passports have been needed for all entrants by air for a year before that.
All foreign national are required to have a visa to enter the US. Even American citizens are required to have a passport to re-enter the country.
Yes, US citizens need a Visa to enter and exit Costa Rica.
yes, all entry into Canada is required to have a passport, you'll need it to re-enter the US anyways.
as of June 1st, yes. You'll need it more for your return to the US then anything else.
Yes, you still need a visa to show that you are a US resident. This is to ensure your safety.
No. American citizens don't require a visa to enter Mexico. All you need is your passport.
No, US citizens do not need a visa to travel to Romania. I myself have been there three times. Romania is a part of NATO. Any country that is a part of NATO US citizens do not need visas to enter. US Citizens need a visa if they wont to travel to Russia. I was in Frankfurt and some American didn't have a visa for Russia. The Airlines flew him to Russia but when he landed the government put him on the first flight to Frankfurt and deported him.
Canadian, American and European citizens can enter Mexico without additional requirements other than their passports.
Canadians can only travel with the country of Canada without a valid passport. A valid passport or a valid permanent resident card are the two acceptable forms of documentation to re-enter Canada without issue.