Give the Duluth Grill a try.  They have nice fresh organic food that is delicious.  In my experience, "Local and organic" is usually marketing slang for "overpriced and boring," but not at the Duluth Grill.  And if you are a food tourist, it was even featured on Diners, Drive-Ins, and Dives.  The banana cream pies are made to order and fantastic.

Try the restaurant on the top of the Radisson it rotates 360 degrees you get a good view the food is good service is great.The hotel rooms are kinda outdated and the room is pie shaped but if you get one with the view of the harbor it's cool.

The Superior Trail runs right through Duluth. It's a moderate-to-challenging hike with lots of breathtaking views, and feels quite remote even in the city. See link for info and trailheads, etc.

To see

If you can you should take a drive up the North Shore and do whatever yanks your crank on the trip.  Watch for Betty's Pies.  No explanation needed.  Lots of stuff to do in town.

I forgot

Forgot my food advice.  There is a place (Northwoods something I think)  It is right to the west where I35 turns into 61.  Good food.  Go real fancy and try the macaroni and cheese and the meatloaf.  LOL.

Definitely would recommend Fitgers!