Paris was much better than I'd hoped and my hopes were very high. For the first 11 days, I had good to terrific weather. The trick was to wear the least clothing that would keep me warm in the chilly mornings, because by noon, it was warm and sunny and even at 10 or 11 p.m. it was not yet chilly.
I'd only been to Paris in June and September-October. Spring is glorious! Trees laden with colorful blossoms, thick grass in the parks (mostly with people sprawled out and enjoying it) and beds of multi-colored flowers everywhere, and aromatic lilacs in full bloom, just gorgeous. I didn't go to as many museums as I'd planned - it was simply too beautiful to spend daylight hours inside.
I went to Reims on Day 12 and it rained. But who cares, the champagne caves were warmer than the outside temps and drier too. One night there was perfect, just enough time to see Reims and Epernay and get back to Paris. After all I still had about 20 sights on my to-do list and only 4 days left. Yikes!
Giverny was spectacular. A riot of color, lots of tulips, not too many people. The only drawback was that it's too early in the year for water lilies.
Lessons learned: do your homework. I did not phone the Chartres Cathedral to verify the Malcolm Miller tour. When I got there (€32 round-trip train ticket), I found out that he was on vacation and would be resuming his tours the next day! I kept putting off going to the Cognac-Jay Museum, and when I finally went, they had just closed for renovations.
Early on in my visit, I found Cafe Martini just a few meters from the Place des Vosges and a 4-minute walk from my apartment. I looked for it because I had learned that it had the best hot chocolate in Paris. Well, it doesn't. Even though the new owner discontinued to chocolate, this very quickly became my neighborhood hang-out. They serve only Italian food and drink, very limited menu but excellent. I drank Aperol spritzes almost nightly (they're closed Sun-Mon), and was introduced to a fantastic red wine from Puglia, about the best I've ever drunk. It's a small place, owner and staff are super-friendly and speak English fluently. The clientele are a combination of locals and tourists. I often ended my day there, as it was on the way home. Okay, I went there, even when I was coming from another direction. So I never tried any of the other restaurant recommendations.
My biggest mistake was tasting one macaron while on a self-guided chocolate crawl (did I mention French chocolates? the best in the world). I'd never tried them because I was sure I wouldn't like them. After that, I must have eaten 50 - no, not all at once, but spaced out between the chocolates :-)
I'm keeping the list I compiled from all your wonderful suggestions for next time . . .