Be sure to take a passport (though that technically isn't needed at the moment for travel into Canada, IIRC) or your birth certificate as a means of demonstrating your U.S. citizenship. In my experiences, getting into Canada wasn't hard, but getting back into the U.S. was a hassle and would have been much easier had I simply had my passport or my birth certificate with me.
Sorry, don't mean to hijack the thread - just thought I'd remind those of you going that the border guards can be a bit hardnosed at times.