I'm also considering going to Giverny during the first week of October. Are the gardens beautiful at that time of year? Or does "time of year" make any difference at Giverny?

Of course it does, that's true for any gardens (except maybe in some tropical places). You wouldn't want to tour gardens in November, for example. They do stay open to the public until Oct 31st, so you can go. They will have annuals until frost.

I had a lot of extra time because of the bus schedule, also. I didn't think it was very well-coorindated at all with the trains, and that was the problem -- and I went some years ago. You had to wait about 45 minutes upon arrival for the bus, also (I got there around noon, as I recall).

I would definitely do daytrips if I were in Paris a week. I really liked Chantilly, Rouen, Auvers-sur-Oise and Fontainebleau, as well as Chartres for the cathedral (although I didn't find it of any particular interest otherwise). If you can handle the logistics, I could see doing a chateau, but that is maybe a pricey daytrip.