We use North Sterling State Park most weekends in June, and some in July/August, depending on the water situation. Mid September the park will not be busy at all.
Colorado State Parks are reservation only now, but in September (assuming not Labor Day weekend) you can just show up and call for the sites when you get there.
All that said, I would ask why you'd add this as a stop? Without a boat, there really isn't much to do. There are a couple of trails (not many) and an archery range. Depending on our water year, there could be very little water in the res.
If you want to see Colorado, go further West into the mountains. Mid September in the mountains is my favorite time of year.