One of the many popular attractions in Porto is the Six Bridges Cruise on the Duero River. It is a boat trip lasting from about 50 minutes to a full hour, which starts either at the Ribeira docks or at the Vila Nova de Gaia quay. However, tourists interested in the history of the bridges in Porto may be disappointed - the information provided by the guide (usually through speakers) is very scanty.
The Six Bridges cruise is a bit like a gondola or a vaporetto cruise in Venice. It lets you see interesting elements of architecture and landscape on both banks of the Douro River from a completely different perspective. As the cruise itself is quite short it is usually a relaxing break in sightseeing, it also allows you to take excellent photos.