I don't know what you are looking for as "hidden" gem. Green Bay is probably one of my favorites, but I don't know if it's hidden.
Fox Cities, not the largest amount of fans, but certainly enthusiastic. The water stations used to have contests amongst themselves and clubs and business actually "want" theirs to be memorable to the runners. And "yes" if you preregister you will have you name printed.
I haven't run the full, but I've run the 2nd half of Paavo Nurmi in Hurley, WI. Although the race is right on county roads, except for the exchange zone, for 13.1 miles on the road, I did not see another runner. I had one vehicle pass by me and there were 4 water stops that I was very happy to see as I approached as I would begin to wonder if I was running my own course. I think the whole town is running the race or manning a water stop. The post-race award party is something to experience at least once, it is so stereotypical small town Wisconsin! It is the oldest marathon in Wisconsin, est 1969, and believe me, there are some oldtimers that probably ran it back then and the course is not for the faint of heart.
I have Grand Rapids on my list to do, just because the race director is soooooooooooo enthusiatic about his race. I got the chance to talk to him on the phone a day after running Fox Cities marathon. He said "I don't suppose you'd like to run another?!!" It certainly sounds like it'd be a good. Pace teams aren't the normal 4:00, 3:30....but beat Oprah, beat P. Diddy, etc... I knew after talking to the RD that I really want to run his race.