Hop on/off bus, and museum pass in Paris

There is no advantage to buying HOHO bus tickets or a museum pass before arriving in Paris. You can buy the HOHO bus ticket from the driver at any stop. BTW, I recommend L'Open Tour which has 4 interconnecting routes. They have a nice website showing maps of their routes.

Buy the museum pass at one of the less busy museums, such as the Cluny -- not at the Louvre, D'Orsay, Pompidou, or Orangerie. Be aware that even though you will be able to bypass the line for tickets, you will still need to wait in line at some museums for security, but the line usually moves quickly. Security involves going through a metal detector at some museums and/or a peek into bags & purses.

Carolyn is right on. Better to buy it in Paris. For an extra 5 Euro you can get a second day on the bus. It pays for itself as you can use it to get around vs a taxi or the Metro. AND....you are seeing Paris from the upper deck while you are getting there. We bought our museum pass at Napoleon's Tomb and it was almost empty. (The ticket office....not the Tomb).

As there can be some confusion about the tourist passes in Paris, I thought this was worth a mention:

Be sure it's the PARIS MUSEUM PASS you're looking to purchase and not the PARIS PASS. The Paris pass includes the Paris Visite transport pass in addition to the museum pass, and the majority of tourists find that it's much cheaper to by the Museum Pass plus a carnet (book of 10) of single-ride tickets for public transport.

Unless a visitor has mobility challenges, central Paris is relatively easy to explore on foot, and is the best way to take in the architecture, window shop, amble through the parks, etc. My husband and I shared one carnet of tickets over nearly a week in the city, and barely used them all. They currently cost 13.70 euros, and never expire; buy them at any metro station.

http://www.ratp.fr/en/ratp/r_61656/t-ticket/

And to echo the others, buy your Museum Pass once you arrive, and at one of the lesser attractions it covers so as not to have to wait in a long line. It may also be purchased at the TI desks at the Paris airports, and some FNAC stores, among other places:

http://en.parismuseumpass.com/rub-the-pass-points-of-sale-3.htm?cat=1&type=41

See the website for the complete list of pass outlets (click "in Paris" for dropdown):

http://en.parismuseumpass.com/rub-the-pass-points-of-sale-3.htm?cat=1

Paris Museum Pass was fabulous - allowed for just popping into some museums for a quick look at pieces of interest (without the feeling of 'having to get your money's worth' by staying longer than you would like) or to use the bathroom. There was still a line at the Louvre (August!) but it moved extremely quickly compared to the non-ticket holding line and was worth it completely. We bought ours before we left the airport when we arrived at Orly, very easy.

If you are arriving at the airport, both the Museum Pass and L'open tour is available here

http://en.parismuseumpass.com/rub-the-pass-points-of-sale-3.htm?cat=3