Aside from the imposing Santa Luzia sanctuary up the hill, Viana do Castelo's most interesting attraction comes in the guise of a real ship ("Gil Eannes") which was used until the 1970s as a hospital ship around the world.
For a mere €2, you can board the ship and visit it in its entirety. It really is very interesting. The rooms are exactly kept as they were, from the bakery, to the kitvchen, to the bedrooms and toilets.
The most intriguing part is also the creepiest. As you venture to the lower deck and down two levels, past the engine rooms and several narrow stairwells, you can actually access some dingy operating theatres and recovery rooms. Again, they're kept exactly as they used to be, with original tools and various memorabilia.
There is atmospheric but discreet music throughout. Also recommended is the scenic view from the deck.
Overall, a different afternoon and an instructive real-life look at how ships used to be in the 1950s and 1960s.