There is much to see and do in this historic and vibrant university city. For a start a bus tour(Hop On-Hop Off) is recommended, but the walking tour is always a better option, not to say that it is free.
The Colleges, 39 in number and spread all over the city, represent its very heart and some of them can be visited. The oldest colleges are University College, Baliol and Merton. If you want to visit the Christ Church College, take your time and stay in line as people come from all over the world to see the place where many of the scenes in the Harry Potter feature films were shot. Some others want to see the school where Charles Dodgson, Lewis Carroll for literature, came to study maths and ended up writing the world famous "Alice's Adventures".
Unfortunately I did not have the patience to spend half a day waiting at the gates, so I missed this opportunity... but people say it is worth visiting.

The Botanic Garden, an oasis of peace and calm and an interesting place even for those who are not particularly keen on studying plants. Go there on a sunny day and enjoy the stroll !

The museums are not to be missed. The Museum of Natural History, for example, facilitates a close up look at the Dodo bird and the complete skeletons of a Eustreptospondylus and a Camptosaurus, which, in spite of the intricate names, look like any other dinosaur to me. But the kids can have a great time here!

Ashmolean Museum, Britain's first public museum, exhibits an impressive collection of art and artifacts: from Egyptian to Roman, from Turner and Constable to Kandinski. My favourite? A David Tindle...

I couldn't find time to reach the Modern Art Museum, which had an interesting (I think)exhibition: "Painted doors".

Oxford Castle, that used to be a prison, is now a luxury hotel. The old tower can still be visited for a 6.20 pounds fee (student discount 10%). As a matter of fact, if you have a student ID, there are lots of places offering the 10% discount.

We spent every night out, going to the cinema or the theatre, some of us enjoyed a baroque or classical music concert in the chapel and all of us ended the day, almost invariably, in a pub (and there were some on our street).