Take a guided/self-guided tour of Christ Church College. Buy tickets in advance!
The University of Oxford's official history museum is one of the best museums in the country. We loved our time visiting here!
Beautiful Cathedral, amazing Roman ruins and the Royal Mile. There is a lot to do here and the drive (or train) to it has a lot to offer as well!
Our favorite restaurant in Oxford. Great food at a beautiful location, right down the road from the well that inspired Lewis Carrol as he wrote Alice in Wonderland.
Can be done either as a "half day" on the way to Bath or a full day. If you plan on visiting Burton-on-the-Water we would recommend getting there as early as possible because tour buses will show up there. Go nearby and visit Lower Slaughter as well, a picturesque town (and when we visited last year at this same time, were having their annual dog show).