Nice or Paris after London?

Hello all! My BF (26M) and I (26F) and I are planning a trip this summer to Europe.

We know we definitely want to spend at least 4 days in London. Then debating whether to do 4 days in Paris or Nice.
I've been to Paris already before in high school and I have always wanted to see the south of France.
But my boyfriend has never been to Paris and as a history teacher I know he would love it.

There's still so many things I haven't done in Paris like the catacombs, Versailles...

Do you have any recs of good day trips from Paris?

We like a mix of tourism and also relaxation.