Godstow Abbey is a ruined medieval nunnery on the Thames north of Oxford, atmospheric precisely because so little is over-managed. Stone walls, grass, river light, and open sky make it feel far removed from the crowded colleges.
It is a fine walking or cycling target from Oxford, especially if you want a softer, rural edge to the city. The ruins also carry literary and historical associations, but the main pleasure is the riverside approach and the quiet of the site itself.