Which city, Płock or Włocławek,
Plock. It is pleasant and interesting. Wloclawek is unremarkable (if you know Scunthorpe, there's a bit of a feel of that).
Plock is one of my favourite places in Poland. There's a lot of history there, and a good cultural base (try the regional museum, the medieval RC Cathedral and also the Mariawicki Cathedral which is unusual). There are also some very good restaurants in the square (and down by the river), and plenty of hotels which are central. In Wloclawek, you won't find the same thing at all.
There's some quite nice countryside and small towns in between which are worth visiting.