First of all, I have to recommend the Mill City Museum to my fellow Twin Cities social studies teachers. Small and manageable, the museum does a good job of providing a coherent (if a bit monochromatic) narrative of the city's history. I'd bring students here for examples of material culture and oral history, or for a unit on urban planning and geography.
The School of Bloggers had a lovely day walking around downtown yesterday to the museum from the central library (which I also hope to make into a field trip someday). At the museum, we found ourselves on a double date with a Minnesota Timberwolves player and his (much shorter) female companion.