What is the best time to visit Niagara Falls State Park to avoid crowds and see rainbows?

Show up right at sunrise or within the first hour after. You’ll avoid the crowds, and the rainbows are actually better in the morning because the sun is lower and hits the mist at the right angle. Crowds start building around 10 AM and peak midday, so arriving at 7 AM (or earlier in summer) gives you the main viewpoints mostly to yourself.