For me, camping season is too short to begin with, and by calling it quits for the season in September, campers are cheating themselves out of at least a month of glorious camping. Most private campgrounds are open until Columbus Day. State parks are open until at least the end of October, and some like Mauthe Lake are open year-round. Campers who visit these places in fall will notice dramatic differences from their summer camping trips. For one thing, the crowds of summer have disappeared and you should have no problem booking a site, even at popular state parks.
Campers also seem more laid back in fall.
Instead of hurrying to the beach or racing around on mountain bikes, fall campers seem content to just soak up the natural beauty of the campground. One of my family's favorite places for fall camping is Door County's Peninsula State Park. We have not been there in recent Octobers, but the memories of past fall camping trips there are still fresh in my mind. When our daughters were young, we took a small passenger trailer that hitched to my bike. Peninsula State Park has beautiful bike trails paved with traffic bond, and you can't find a more scenic place in Wisconsin to go pedaling.