Belle Isle is a classic, timeless location with lovely architectural details and many photo options: 

 

The fountain is typically turned on from Memorial Day to Labor Day, but it is a beautiful location whether the water is on or off, as you can see from the photos below. 

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Sunset Point is a gorgeous spot with sweeping views of the river and the Detroit skyline. It's dramatic and beautiful, and especially suited for when the sun sets over the Detroit skyline. 

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The historic casino building has lots of photo opportunities between the columns and on the little terraces. It's especially beautiful around sunset, when the light streams in golden through the pillars, with the Detroit skyline in the background.

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In addition, there are a couple of lovely gardens and trails.