First off – take a look at morcmtb.org if you haven’t yet already. Memorial Trail in Red Wing I believe is one of the definitive DH trails in MN (it’s pretty flat area around here. There are some more options up north a bit, depending on where you are. East of the twin cities is Afton Alps which has some DH as well (but they charge $9 entry fee by the way). I don’t really know what’s in the north of Minnesota.