Unfortunately, the Oxford Story (which would have also been close to the top of my list of recommendations) closed down at the end of October last year.
I'm not sure that the 'Oxford Classic Tour' buses are running any longer either. The only open top bus service, on the city's official website, is now the City Sightseeing service, which uses red buses:
http://www.citysightseeingoxford.com/
If the weather's good, the University Botanic Gardens are lovely:
http://www.botanic-garden.ox.ac.uk/Garden/gard enintro.html
For further ideas, browse here:
http://www.visitoxford.org/see.asp
Chris