The Oxford Botanic Garden lies at the end of High Street, across from Magdalen College. Tulips and other flowers burst into bloom in May, which is when I visited. Beside the garden there is a pathway that leads to Christchurch meadow. Beyond that lies the Thames River (called The Isis in Oxford), where you can see Oxford crew teams practice.
For history, hours and visiting information: http://www.botanic-garden.ox.ac.uk/