I don't know whether you like this kind of thing, but I know where I would be heading with a day to while away in Cambridge:
Fitzwilliam Museum and/or any of the many other museums .

The smaller museums are often only open in the afternoons. More than enough Bookshops to Die For to spend the morning (or several days) in ! Heaven.

Have a great time, whatever you do. It's a lovely place.

- jeanie