Wonderful trip! I strongly recommend several of the Optional Experiences to make the most of your time, including the sunset boat trip in Paris (you see the Eiffel Tower aglow!), walking tour of Hidden Venice, and walking tour of the Colosseum in Rome.
Be prepared to walk a lot (I hit 10-14k steps most days!) Be prepared for early mornings.
These are full days packed with lots of information, the bus and all the people in it become family. You don’t have to be fit but there is a lot of walking. They help to get you into places where you’d otherwise have to queue up for hours. Amazing scenery