The Walker Art Gallery is a great place to visit. Nice range of pictures from the Medival period through to the present day. Its mainly over one floor, so is not so big you can get lost, but still put by an hour and a half to 2 hours to do the gallery.
A good range of famous artists, from Rembrandt, Matisse, Turner and so on. A real gem and like all good galleries its free admission with a suggested donation.