Well, in a very non inclusive nut shell - pretty much everything in the metro that is of interest to a visitor is on the 800 MHz P25 Trunked system... not really anything of major interest on UHF and VHF has some fire dispatch simulcasts...
Big Lake is in Sherburne Co, which still uses conventional VHF. Their frequencies should all be listed in the RR Database under Sherburne Co. It's also right on the border with Wright Co, which also still uses VHF.
You might also try listening to fire dispatch from Anoka Co (154.280) and Hennepin Co. (154.385) These counties are digital but have their fire dispatch patched to VHF. They will be more active since they are more populated counties in the metro area.
Also, just FYI...Having a scanner in your vehicle is illegal here in MN unless you have a ham radio license, so keep it well hidden if it's in the car, preferably in the trunk with no batteries in it.
I'm not entirely sure about Rochester. I know some of the area has at least started to make the switch to digital, but I don't know any specifics.
As far as having your scanner in the car...I just wanted to make sure you know it's illegal. Personally, I had a scanner in my car for several years before I got my ham license and never had any problems. You aren't going to get pulled over just for having an antenna on the car.