The 34-year old Renaissance Center, or Ren Cen as locals know it, is the headquarters of General Motors and home to the Detroit Marriott, the tallest hotel in the country. Though its retail ventures faltered, it's a successful office complex. Ren Cen is certainly a geographical and historical landmark, but the reality is that there's not much to do inside. The exception is the Coach Insignia, the lone restaurant that sits atop the center. It's open at night and worth a few dollars for a beverage if only to see the view. Not that the view is that dramatic -- it's just fun to see Detroit's wagon-wheel shape. Outside, the 14-acre campus is the start of the Detroit Riverwalk, courtesy of The Riverfront Conservancy. This concerned group of ecological investors, with deep pockets and good connections, turned a desolate pile of rocks and chewed-up landscaping into a wide, asphalt walkway 3 miles long. Flanked by tidy landscaping, the Riverwalk is a lovely backdrop for Detroit River Days, held the third weekend in June, when Detroit and Windsor celebrate their independence days with fireworks and festivities. For an entrance fee of $3, you can concerts, boat tours and sand sculptures, among other family-friendly activities. Farther along, the Riverwalk enters the William Milliken State Park. Flanked by tall seagrass and overflowing pots of geraniums, it's a great spot to rent a bike, ride the Great Lakes theme carousel or take out a dinghy. Boaters rejoice -- the tiny park even holds a marina.