Well there are a few places here in the Green Bay area to ride. First of all is Baird Creek Park....most riders go there. There is also the Reforestation Camp (NEW zoo area) I love going there..lots of singletrack. Being I"m still new Baird is a bit too technical for me..but it has places for beginner to advanced.
Hope this helps. There are other areas around too..LIke Kewaunee, Calumet, etc.
Do you compete in any WORS races?