The best view of famous Cologne (Köln) Cathedral is from the Rhine (or across it), but you’ll probably be right in front at the Cathedral square. This church is BIG, people don’t realize how tall it is. Get back as far as you can – you can get halfway across the street by MacDonalds before the traffic lights, lamp posts, etc. intrude across the Cathedral itself. Even then you’ll be tilting your camera upwards.
Walk round the Cathedral outside and pick out elements of the architecture you find appealing, there’s plenty to choose from ! On your way, check out the Hauptbahnhof (main station) – it’s quite impressive.
Inside the Cathedral, it’s huge size makes it spectacular, but of course being Protestant it is not so richly decorated as many Roman Catholic churches.
When I was there, you could shoot from the back of the Nave but the actual body of the Nave was closed off, as were the side aisles. Use the pews or columns to support your camera during the slow exposures.
To get to the Shrine of the Three Kings you have to exit the Cathedral and enter at the South Transept (halfway down the square side) at the Pilgrimage entrance. More low light here, so you’ll need to find some support for your shots again. The large silver candlesticks are worth a shot as well.