Tall ships normally do their stop in August. Definitely do a harbor dinner cruise if you've got the time. I like the north shore of Superior for the views of the lake. Keep an eye on the shipping news to see when ships are coming and going out of the harbors so you can go check them out.

http://duluthshippingnews.com/

If you can ever swing a trip through there in the summer, head up the north shore then up to Ely for the Blueberry festival.  Your wife will really enjoy it.

https://www.ely.org/events/blueberry-art-festival

You can also stop at the Wolf Center there.

Then head west out of Ely to check out the Lake Vermillion-Soudan Mine.  Really cool tour.  They take you deep under ground and turn the lights off!  Really makes you think about what those miners dealt with.

http://www.dnr.state.mn.us/state_parks/lake_vermilion_soudan/tours.html

They do particle physics research there now...at least I think they still do.