Head over to Cannelle, grab a coffee and breakfast pastry, then go enjoy them a few feet away in Capitol Park. Then go for a short walk. Walk to Washington Blvd (2 blocks west), then up the path in the median to the Book Building that houses the Roost Hotel. Check out the restored lobby.
By the way, La Supreme (inside the Book Building) is perhaps the best restaurant in downtown Detroit. They do brunch on weekends, otherwise it's dinner only.
Continue your walking tour by heading north again on Washington Blvd to Grand Circus, and enter the lobby of the David Whitney Hotel. Take a few pictures, etc. Then walk south on Woodward to enjoy the architecture as you approach Campus Martius. Maybe have a meal at Parc in Campus Martius.
Then stroll a little further to see the famous lobby of Detroit's Guardian Building (red brick skyscraper), which is essentially where the Great Depression started. If you want something quirky, head here: time.com/collections/...