Central and Northern Minnesota is a vast area. Pretty much covers about 2/3 of the state.
You might get better and more specific suggestions if you can be more specific as to where you might want to be going? What do you like to do and what do you want to see?
Duluth? North Shore? Detroit Lakes? Bemidji? International Falls/Canadian Border? Northwest? Central?
Each of these sections of the state have something to offer. Great State parks throughout.