Oxford is clearly dominated by the university. THe heritage is unbelievable. The old buildings that one can look around, I recently visted St. Edmund Hall the buildings were buitiful, very touristy. There are so many colleges to visit that alone could take up a serious amount of time. However Oxford also has many other attractions, such as museums, an ice rink, laser quest and many delightful shops. The main problem however is parking, don't even attempt to in the centre. You need to take the Park and Ride bus to have any oportunity of not wasting your entire day.
For food during the day there is a McDonalds but if you just take time there are may very nice sandwich shops that sell reasonable priced food that is far more healthy. I also had the best pizza i have ever had in oxford after the millenium, anyway have fun visiting Oxford