There are few hotels located right in the centre, but if you are lucky enough to stay in one of these establishments (e.g. The Randolph, the Old Bank, etc.) then all the main sights are within a stone throw.  Buy yourself a map, as the heart of Oxford is a veritable maze of narrow streets, all with lots of interesting places.   Get a ticket for one of the tourist buses, hopping on and off at leisure.  If you are just interested in the main sites, you will not need to use public transport.  Indeed, at peak times it may take you just long by bus from Carfax to Magdalen bridge than on foot!

If you are not staying in the centre but in some of the hotels or guesthouses on the Banbury Road or Woodstock Road, there is a fast and frequent bus service to the centre, although at night (especially late) you may prefer to use taxis.

If you visit Oxford in the summer you should definitely go punting.  You can hire punts either at Magdalen bridge or Folly Bridge.