If you have a lot of time in Oxford, some of these are well worth a visit perhaps as a half-day/full-day trip from Oxford.
I. Blenheim Palace
Blenheim Palace is located in Woodstock, 8 miles north of Oxford. It is one of the largest private palaces in England, located in a 2000 acre estate. Sir Winston Churchill was born here. Currently it is the residence of the 11th Duke and Duchess of Marlborough. The palace, garden and park are open to the public and offer plenty for a day out for the whole family. This is also the site of Blenheim Palace International Horse Trials.
The Cotswolds, a stones throw from Oxford has been designated as an area of outstanding natural beauty (AONB). Dotted with picturesque countryside and villages, this is certainly another day-trips worth.
Stratford-upon-Avon, an hour north of Oxford by train is the birth place of William Shakespeare, the Bard of Avon. It is now the home of the Royal Shakespeare Company and one can watch many of their productions here. Shakespeare's house is also another famous attraction. There is plenty to do here including a cruise on River Avon and museums.
V. Thames Path: Cycle/walk to Iffley lock
The 300km Thames Path National Trail that hugs River Thames passes right through Oxford. The path is very scenic and one comes across numerous locks that control the level and flow of river Thames. Walking towards London along the path accessed just after Folly Bridge, one reaches the lovely Iffley village and Iffley lock. During University term time, this bit of the path is abuzz with boat crews preparing for boat races.