I have traveled with Globus,not Cosmos but many of the optionals are the same so I will try to answer your questions.Cosmos is owned by Globus.
Rhine River Cruise--- Absolutely wonderful,do not miss it. Because of this short cruise in 2004 DH and I took a rivercruise on the Avalon Tranquility from Budapest to Amsterdam last summer for 16 days.Globus also owns Avalon. It was fantastic!!

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Cable car----Yes IF the weather is good.
Louvre----We did this on our own for about 1/2 the price.The Museum D'Orsay is very near the Louvre and worth seeing too. We also did that on our own.I prefer to do museums on my own as I want to spend more time in them then the tour allows.
The busses are fine,very comfortable with hugh windows for sightseeing.. . We skip most of the optionals as we like to do things on our own but we have traveled to Europe 5 times.Most people do almost all the optionals particularly if it is their first time in Europe. Many times the TD will have the bus drop you off in the city central and pick you back up or give you directions on getting back to the hotel. Otherwise the public transportation is excellent in Europe and armed with city maps,bus routes or subway maps it is very easy to make your own way wherever you want to go.If you just do a little research on each city you are going to on Tripadvisor or Frommers and pick up some guide books it will be simple to determine the places you would like to see on your own and maybe even find some good restaurants away from the group.. .
I have taken public transportation,either subway or bus or boat in London,Paris and Amsterdam and Venice and all are very easy to find your way around and use the public transportation.
DH and I are headed to Italy for Rome,Florence ,Tuscany and Venice in May and we will be doing many things on our own.