In the UK, Wells is a lovely town, good for one day.
I looked over your list and saw Bologna in Italy missing. It's one of my favorites, easy to do in one day. GREAT food! Neat university. Lots of young people around.
You haven't seen Berlin, according to your list, but that could take more than one day. I also think Salzburg in Austria, birthplace of Mozart, is very cool and totally doable in one day.