We did a one night camping trip for my birthday at Cart-In Site 1 the second to last weekend of September.
We loved the hiking at Frontenac State Park - walking along the lake and then walking along the bluffs at sunset was magical! Fantastic views. I imagine if we had gone two weeks later, we would have had a fantastic display of fall color. We went a little too early to catch the changing leaves.
We loved how easy Frontenac was for a casual weekend getaway. It was an easy, scenic one hour drive from St. Paul that was very enjoyable. The park was also nice with great bathrooms at the main campsite, nice trails, and they even had board games to check out at the front office for a rainy day- a great idea!
We stayed at Cart-In Site 1. I would rate the site itself a 3. It was very secluded and spacious, which was fantastic! We could not see any other campsites from our site, which was fantastic. We only occasionally saw some hikers on a nearby trail. We could hear some other noisy campers, but it wasn't too much of a disturbance. Very wooded, nice, spacious site. There is not water at the Cart-In parking lot, so be aware that you will have to fill up prior to parking and heading to your site (easiest place to do so is the main office/ visitor center).
I also slept poorly because of a nearby train that kept going past and honking at least until midnight. So if you're a light sleeper, be aware of persistent train noise at these sites!
Overall good site and an easy, fun, and relaxing weekend getaway from the twin cities.