One of the coolest things to do in Hamburg, the Old Elbe Tunnel allows you to walk under the river. Which doesn’t sound all that thrilling, but seeing as this tunnel was an architectural wonder when it opened in 1911 and the walls are lined with beautiful ceramics sporting nautical themes, it’s a great free activity. Unfortunately the other side is not interesting, but there are some good views back to the main part of Hamburg. You’ll also get an up-close look at the tunnel’s inner workings. The tunnel is about 420 meters long and you are about 24 meters underground.