Plenty of unmanned border crossings bro. Just go native. 
It used to be that you could just go through a turnstyle at Niagara Falls, and the only folks who would question you were the US Customs guys on the way back in.
Go through the turnstyle, walk across the bridge, smoke a Cuban, and walk back. It was a good way to wrap up your day after some business and a nice dinner and drink at the Hard Rock cafe.