Thanks for your suggestions. Yes, I will get a guidebook in order to finalize the list of museums.

Picasso Museum is still closed for renovations.

Oh yeah you are right about Picasso-I see they are coming in August.

<i>Regarding the ticket t+, what is the validity of one ticket </i>

One ticket t+ is valid anywhere on the métro (and RER within zone 1). You can make as many connections as are required to reach your destination.

One ticket t+ if valid on most all buses (the Night buses, Roissybus, Orlybus, and 350/351 are most of the exceptions). Connections may be made as long as the direction of travel is respected (no doubling back) and boarding any specific bus number can only be done once. What is not allowed is disembarking, bus 87 for example, and re-boarding a later bus 87.

Connections, within these guidelines, must be completed within 90 minutes of the first boarding.

Connections from bus to métro are not allowed. Connections from bus to tramway are allowed.

This section of the RATP.Fr site should answer most, if not all, of your questions:

http://www.ratp.fr/en/ratp/c_21879/visiting-paris/

Wow! Very useful feedbacks. Thanks a lot.

I don't know of any museums that close on weekends.. but that doesn\t mean there aren't some.. as I said.. the big ones like Orsay and Louvre close either Monday or Tuesday.. same with Versailles.. check website. .nothing worse then going somewhere .. being all proud of yourself for naviagating the bus or metro to get there.. and discovering its closed!!

As for top five.. its so hard.. as others say its a personal taste thing. Even within the list of my personal favorites.. it may only be certain parts of them.. places like the Louvre are so huge.. you could love one area and have another bore you.. I have been quite a few times.. often going twice per visit.. because its a lot to take , and I notice that people who say its "too big" or it "overwhelmed them " are the ones that try "to do" it in two hours, trying to hit all the major exhibits in one fast visit. Best to pick a few things or sections.
Look up the websites for a few museums.. it will help narrow it down for you ..

Personally I love the Louvre and can't imagine "not bothering" with it, I also recommend the Orsay, Orangerie, Cluny, Rodin ,Invalides Army Museum , Shoah Memorial Museum, I personally am not a person who enjoys modern or abstract type art.. but there are those who would never miss seeing that type of art.. so there is no right or wrong.

National museums close Tuesdays. Municipal ones close Mondays.

I am in the not a Louvre fan camp, but go for a couple of hours at least to explore "something". Unlike the Orsay which I never tire of.
The OP might be interested in the Orsay's furniture collections. Beautiful.

I totally agree with Gretchen. In fact, in my last 10 or so trips to Paris I haven't even set foot in the Louvre. I can always make time for the Orsay or the Orangerie or the Marmottan, though. It's probably not fair to make comparisons, though, as I have likely been to the Louvre at least 30 times over the years. It's just not a must-go place for me anymore. Plus, I live in Washington, DC, most of the year, and spend a LOT of time at Smithsonian museums and the Phillips and Corcoran, etc.

Thanks all for the recommendations on museums.