Perhaps a hike to the end of MN point, past the old buildings. Go up Congdon park, climb the Rock Knob at Hartley, go up to Stoney Point, or along Seven Bridges Road, eat at Hugos, Sala Tai, Lotus Inn, Duluth Grille (although I suppose that doesn't really count as it was on TV) or Italian Village. A movie at Zinema. Poke around Central Sales. It's not "quirky" but the Tweed has a really good exhibit right now, or go to the Karpeles. See if you can talk to Dick Rosvall and look around his place/studio.

"This is probably obvious, but bring them to the Anchor Bar.

Vicarious"

The comment has it's virtues, the Anchor is grand. One extra virtue should not go missed, I might add! See, you've already started to pick up the flavor of Duluth. Misuse of 'bring' and 'take' is rampant here, and Duluthians are utterly incapable of understanding the difference even when it is explained clearly.  The proper thing to say would be 'take them to the Anchor Bar', unless it's being spoken by someone already at the Anchor Bar.

Central Sales is a store that has odd items sort of like a combo of a thrift store/hardware store.  You can find items no one else has.

Please don't feed the seagulls. Really.

Go check out the S.S. Meteor - she's sorta the William A. Irvin's older, secretly more significant, down-on-its-luck crusty hipster cousin.  Might want to call ahead to find out which days they're actually open - it's usually pretty quiet which means you might get a one on one tour with a very knowledgeable and salty tour guide.  

If your friends get a kick out of her you can take a quick driving tour afterward and show them three almost totally ignored and very historically significant lakers that are languishing a mile or two away:  the retired J.B. Ford (107-year-old freighter that looks almost exactly like it did when it entered service in 1904) visible to the east of Main Street on Connors Point, docked just ahead of all those tugboats; the laid-up American Victory (built as a oceangoing Navy tanker during WWII, served in the Pacific Theater and shot down enemy aircraft before being converted to a laker) towering just to the west of the same stretch of Main Street, and the laid-up Edward L. Ryerson (art-deco naval architectural masterpiece / iron ore carrier) sitting proudly at the intersection of E 1st St and Cumming Ave.

http://www.boatnerd.com/pictures/historic/perspectives/JBFord/default.htm

http://www.boatnerd.com/pictures/fleet/amvictory.htm

http://www.boatnerd.com/pictures/fleet/elr.htm

I both earned and deserved that grammatical burn. 

But, wait! I was at the Anchor Bar when I wrote the comment, so I was grammatically correct!

(No I wasn't.)

If they are here on a Saturday, the Saratoga has amazing jazz shows w/o cover. We just heard Tanya and Greg Moore perform yesterday afternoon, it was fabulous! Friendly, friendly crowd, and the place is a trip.

Zra, why shouldn't I feed the gulls at Brighton Beach?

Feeding the gulls is bad. Period. They become a nuisance to others by teaching them that humans are food sources.

I am not a naval architect (IANANA) but my understanding of why the Ryerson has spent much of the 21st century in mothballs is that:

1.  It's a steamer, rather than diesel-powered, thus it's less fuel-efficient/more expensive to operate.
2.  Part of the reason we all think it looks so sweet (not all the reason, mind you) is because it lacks a self-unloading boom, so it can't work some of the ports on the lakes that it otherwise might. 
3. "the bulk carrier was built with one cargo in mind; that being iron ore.  Her basically square boxed holds do not favor the carrying of such other bulk cargoes as coal, slag, or limestone due to the difficulty in unloading these cargoes.  Any thoughts of conversion to a self-unloader would be even more difficult and expensive as her holds would have to be reconfigured with sloped sides to allow for the free movement of the cargo to the self-unloading conveyor(s)."

Bing!

Take them to the new Duluth Art Institute exhibition, which opens tomorrow.  The artist, Adam Swanson, does some very cool stuff that your friend might dig.