San Francisco is one of my all-time favorite destinations with a GT kiddo, but Paris or London is also an amazing and rich environment.
Why is SF one of your all time favorite locations? It's one we get to quite often because grandparents live nearby. Took DS to the Explorotorium last fall, and we like the new Science Museum. And I enjoy several of their art museums. We tend to drive through SF a lot, and see it one day at a time. Is there something special you like there?
We did London when my son was in Elementary. Would love to do Paris, but it's out of our budget right now.