Ushaw is a former Catholic seminary outside Durham, now open for heritage visits, exhibitions, events, and garden walks. Its chapels, halls, and college buildings show a grand institutional side of County Durham's religious history that is quite different from the cathedral precinct.
The interiors are the main reason to go, especially if you like Victorian ecclesiastical design and Pugin-era detail. It is close enough to Durham to work as a half-day addition when you have already seen the cathedral and castle.