We did a river trip with Stern und Kreis, one of the many companies offering River Spree boat trips. We went on the one hour round trip starting and ending at Nikolaiviertel costing 11 euros per adult. it's a delightful way to spend an hour on a sunny day and seeing all the main sights of Berlin from the water, but you don't get a huge amount for your money. The audio guide, included in the price, didn't work so we didn't always know what we were seeing. Oddly, they had the commentary through the loudspeakers on the boat in Spanish - so some of the tour visitors could understand it but not the majority. There are plenty of other provider of these trips so probably worth shopping around - there are probably better offerings than this one.
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