Duluth Sky Harbor KDYT. On the spit that protrudes into Lake Superior.
Detroit Lakes KDTL. Go there on a Friday afternoon, camp the night and enjoy a "Saturday at DL" get together, held every Saturday morning.
In South Dakota, Mobridge: KMBG. Very friendly young couple operates the airport. Inexpensive gas, including mogas pump on the field, courtesy car, nice turf runway, and they let a number of us camp next to the turf runway enroute to OSH. Let us use the bathrooms, etc.
A whole bunch of other possibilities out there as well. Stay away from the larger airports for the most part.
Here is a great resource for the state of Minnesota:
http://www.dot.state.mn.us/aero/airportdirectory/ They do a pretty good job of keeping this up to date. If you scroll down, there is a notation of whether under wing camping is permitted at each airport. Also gives contact info for airport manager, so you can call in advance to learn more about camping, things to do in the area and whether a courtesy car is available. MN airports usually have a courtesy car.
Most of all, have fun.
MTV