You can still have a wonderful rooftop view from Galeries Lafayette.
"Modern" art means many things--Pompidou may cover that better. Orsay is wonderful for Impressionist era.

I wouldn't choose Giverny as a day trip in October. We went last year in late September. It is the very end of the season and is not at its best. We love Chartres as a day trip and it should be lovely in October.

Museums are something no one can really direct you to, go with what you love. I wouldn't miss out on the Pompidou. Then again, I don't miss out on too many art museums.

Pick one if one is all you want and know that the city itself is art on a walk.

Hi mrt,

Here are some suggestions from Fodor's on what to see and do in Paris: http://www.fodors.com/news/summer-20...ef=news_063015. Of course, this list is specifically for summer, but I think you'll find some of the ideas applicable to your trip, particularly for Versailles and especially if you're looking to spend some time outdoors. Hope this helps!