You’re going to have to do some of your own research, but I’ll start, and maybe someone else will jump in. There was no line at the Chagall Museum in Nice. There is also a Picasso Museum in Antibes.

Paris Museum Pass

It depends how devoted you are, but the Paris museum pass covers a lot, and you can get it for different amounts of time.

For Florence, you can purchase advance tickets online but I think they cost more.

Museums are sometimes closed on Tuesdays, it’s good to check beforehand.

The Prado in Madrid is a must. And Picasso’s Guernica is now at the Reina Sofia.
Madrid Card

You should also look at http://www.gomadrid….
for other passes. The Prado, as the Louvre, is very large and to enjoy it fully will take more than one day, so we got a pass that allowed multiple visits. Some passes require you to give them a photograph, so carry your extra passport photos with you.

For Granada you might want to book your Alhambra admission online. There are tons of sites that sell tickets, but this is the official site:
http://www.servicaix…