I gleaned the following from the internets:
Get to sites, like Michelangelo's David, at 0600 or 0700 to beat the lines and heat.
According to USA Today, the cities are "jam packed like sardines," but the countryside is less congested. Get off the well-worn path.
Weaker Euro benefits U.S. travelers.
Book very early if you want to get into the super popular sites, like Versailles and the Eiffel Tower.
If you can, book for September-October when it's less crowded with good weather.
Daily showers show consideration for your fellow travelers, especially in crowded situations indoors where there is no breeze.
The European Travel Commission predicts tourism will be down 20% from 2019 levels, so not nearly as sardine-packet as pre-COVID, or in other words a perfect summer to visit Europe and its environs.