The biggest challenge is that everything is booked. However, there are many first come first served sites at CGs at the Northern Highlands American Legion State Forest in northern Wisconsin. We are staying in one right now, and we drove up yesterday and got in. But they do have reserved sites too, and I would suggest trying to get a reserved site at Clear Lake, Musky Lake or Crystal Lake. These places have good fishing and swimming. There are many smaller CGs in the system, and they will have fishing but not always much else.
You can google their site and look. One big consideration, these are all dry-camping only. No hookups. The CGs I mentioned have modern bath and shower buildings, though. The others have only vault toilets, but you can drive over to the main CGs and use their showers & dump stations. So it's not for everyone, but at least you can get in. The ones in the southern half (Clear Lake area) are all very close to Minocqua, which has a lot to do. The northern half is in a more remote region.
The sites are all fairly rustic, and some are pretty small and/or not level. Finally, this is not the place for anything bigger than 27 feet or so. The CG roads are tight and so are many of the site accesses.
