This should be obvious perhaps, but I'd like to recommend it anyway; take a short walk outside the old town and enjoy the view at the church and the old town walls.
The Stedelijk Museum (town museum) is housed in a beautiful white old house on the main street. Well, it looks like a house. Part of it is in fact a former catholic hidden church, founded here in the 17th century when catholics were officially second-class citizens. The church is preserved as it was and is worth a visit alone.
The Grote Kerk, or Great Church, was built as a catholic church in the 15th century but has been protestant for almost 450 years now. The tower has a gate underneath which runs paralel to the road, square on which is the entrance to the church. See the picture to understand what I mean, my English isn't sufficient. Either the road was busy already back then and they did this for safety, or it is a smart way to make the church a little more defendable. After all it was at the edge of the old town and Vianen wasn't very popular with its neighbours in the Middle Ages.
Prins Bernhardstraat 75, , Vianen, Zuid-Holland 41
Good for: Solo