I've found each cruise line does their own thing excursion-wise, but most all go to the Baths(Nor, Carv, Princ, Diz). Hard numbers are hard to predict, but trends suggest early or late (>4p) are the sweet spots for high season. Someone else here may have a well beaten path to determine crowd levels, but not I.
My SOP is to hit the Baths early (8a-breakfast early) and enjoy them as long as I can before the crowds overwhelm. Then go wander Copper Mine or another feature of VG, possibly even make a run to Hog Heaven for Lunch. Then I may head back to the Baths late afternoon when the cattle-call crowds are on their way back to their respective vessels.