The best tip is to act like a local, don't go out looking like you have you pockets full of greens.
Other than that, bring a good English/Portuguese dictionary and have fun!
If you could team up with a Brazilian your vacation would be way better, cause you don't want to waste your time in some boring tourist cruise, a local could show you the right spot to have fun.
Last Carnaval we received guests from Holland, Turkey and Germany, we had a blast. Maybe next year we'll head up to northeast, like Jer said... Fortaleza or some other HOT city up there!
