You have received some great advice. I was reading through the posts and agree that these 3 comments are very important: keep hydrated, take some photos but don't see the whole trip through the "viewfinder", and don't worry about not seeing everything. The best memories will often be of the things you didn't expect to see or happen. A few things I would add:
Can't stress enough the importance of wearing comfortable shoes and clothing.
If there are any medications that you think you might need, be sure to pack them, or at least the label from the package to show a pharmacist the ingredients. Some are difficult to find. (sounds emabarassing but, try asking a shop keeper/pharmacist who only speaks Italian for something like preparationH or laxatives!)
When touring Versailles, good advice see the Gardens first. Plan an entire day there, at least. We spent a day touring the gardens then stayed the night in the city of Versailles and were able to get up and be among the first in line for tickets to the chateau. That second day we saw the chateau and the fountains, then headed to our next destination that afternoon. We took the tram tour of the gardens which included the Hammeau and Petite Triannon and was well worth it! Also, the paths there, and at many other Paris parks, are are a crushed stone that is very dusty. My black shoes and capris looked terrible within an hour. You can dress for Versailles like you would for a US national park or monument. Next time I will dress for biking, boating and serious walking. Versailles was, unexpectedly, one of our favorite places.
Postcards are inexpensive, easy to pack, and are often better photos than you can take (good examples: the Pieta at the Vatican, Mona Lisa, arial views of anything) Spend a few minutes getting postcards in each city and fewer minutes behind the camera.
Savour each moment and have a wonderful time!