Not sure how that would work if you don't have a US visa and want to flagpole. I've taken the Tunnel Bus before, and US Customs is at the inbound side, where everybody in the bus gets off and gets processed. The spot where you get back on the bus to go back to Canada is over near the duty free and Mariners Church, outside of the US Customs area, way on the other side of the plaza. There may be some issues with not being able to get back over there, since you are not officially entered into the US if you are flagpoling.
If you are able to get to the US, by either being visa-exempt or having a visa, this would work. You could head over to Greektown for a while then come back. But you would have to miss a bus or two once you get back to Windsor, as I doubt they would wait for you to finish landing.
My wife and I live near Windsor, and when we get our COPR, we're just going to take a long weekend trip to Niagara and do our landing at the Rainbow Bridge. Gives us a chance to get out of town anyway. Even though Windsor is closer, I'd rather do it at Niagara.