I have a fake and I am from canada (drinking age 19/18) the fake works beautifully in the states but you'll need some sort of id to get in anywhere. Props on the being able to buy alcohol in grocery and convenience stores though.

Go to mexico. They have no laws at all.
Seconded, was there during spring break, since your under 21 with no fake I highly recomend it. The clubs are full of gorgeous american girls which would be my only motivation for going anyway. Beachs are nice to lie on to get over hang overs as well!