Hull Holy Trinity Church must count as one of Hull's biggest "hidden treasures". It is huge, right near the centre of the town and when I visited it was ringing its bells as loud as it could. And yet there was hardly anyone inside. This is sad. It is a very lovely place. I reckon the stained glass windows rival the best in the country. I took some pictures there yesterday which I think turned out quite well.
If you are in the middle of town and fancy somewhere nice to just sit (they serve coffee and biscuits too) then it is great place to go.